RUST-1076 Remove conditional module definition

[patrickfreed]

RUST-1076

This PR removes the macro that conditionally defines all of the drivers modules. Its intent was to make the compile error issued when a user selects more than one async runtime more readable, but it has the side effect of making the docs.rs page for the crate very noisy. After removing the defines, the compile error is still readable, though it does have some extra distracting info.

example:

cargo check --no-default-features
    Checking mongodb v2.0.0 (/home/patrick/mongo-rust-driver)
error: one of `tokio-runtime`, `async-std-runtime`, or `sync` must be enabled; either enable `default-features`, or enable one of those features specifically in your Cargo.toml
   --> src/lib.rs:376:1
    |
376 | / compile_error!(
377 | |     "one of `tokio-runtime`, `async-std-runtime`, or `sync` must be enabled; either enable \
378 | |      `default-features`, or enable one of those features specifically in your Cargo.toml"
379 | | );
    | |__^

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/client/session/mod.rs:24:5
   |
24 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/cmap/worker.rs:37:5
   |
37 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/cursor/common.rs:23:5
   |
23 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/runtime/stream.rs:19:5
   |
19 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
 --> src/sdam/message_manager.rs:5:5
  |
5 | use crate::RUNTIME;
  |     ^^^^^^^^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/sdam/monitor.rs:17:5
   |
17 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/sdam/srv_polling/mod.rs:15:5
   |
15 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0432]: unresolved import `crate::RUNTIME`
  --> src/sdam/state/mod.rs:48:5
   |
48 |     RUNTIME,
   |     ^^^^^^^ no `RUNTIME` in the root

error[E0425]: cannot find value `RUNTIME_NAME` in this scope
   --> src/cmap/establish/handshake/mod.rs:133:82
    |
133 |                 Some(format!("rustc {} {} ({}) with {}", version, channel, date, RUNTIME_NAME));
    |                                                                                  ^^^^^^^^^^^^ not found in this scope

error[E0425]: cannot find value `resolver` in this scope
  --> src/runtime/resolver.rs:41:19
   |
41 |         Ok(Self { resolver })
   |                   ^^^^^^^^ not found in this scope

error[E0412]: cannot find type `Interval` in this scope
   --> src/runtime/mod.rs:150:57
    |
150 |     pub(crate) fn interval(self, duration: Duration) -> Interval {
    |                                                         ^^^^^^^^ not found in this scope
    |
help: consider importing this struct
    |
11  | use tokio::time::Interval;
    |

warning: unused import: `net::SocketAddr`
 --> src/runtime/stream.rs:2:5
  |
2 |     net::SocketAddr,
  |     ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0392]: parameter `T` is never used
  --> src/runtime/join_handle.rs:13:33
   |
13 | pub(crate) enum AsyncJoinHandle<T> {
   |                                 ^ unused parameter
   |
   = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`

error: aborting due to 13 previous errors; 1 warning emitted

Some errors have detailed explanations: E0392, E0412, E0425, E0432.
For more information about an error, try `rustc --explain E0392`.
error: could not compile `mongodb`

To learn more, run the command again with --verbose.