Revert "Support custom indexes"

[jyn514]

Reverts #1140

This caused a production outage:

Oct 30 15:42:52 docsrs cratesfyi[8350]: Error: initialising registry index repository
Oct 30 15:42:52 docsrs cratesfyi[8350]: Caused by:
Oct 30 15:42:52 docsrs cratesfyi[8350]:     Actual 'origin' remote url "https://github.com/rust-lang/crates.io-index.git" did not match desired one at "https://github.com/rust-lang/crates.io-index"
Oct 30 15:42:52 docsrs cratesfyi[8350]: Stack backtrace:
Oct 30 15:42:52 docsrs cratesfyi[8350]:    0: failure::backtrace::internal::InternalBacktrace::new
Oct 30 15:42:52 docsrs cratesfyi[8350]:    1: failure::backtrace::Backtrace::new
Oct 30 15:42:52 docsrs cratesfyi[8350]:    2: docs_rs::index::Index::new
Oct 30 15:42:52 docsrs cratesfyi[8350]:    3: once_cell::imp::OnceCell<T>::initialize
Oct 30 15:42:52 docsrs cratesfyi[8350]:    4: <cratesfyi::BinContext as docs_rs::context::Context>::index
Oct 30 15:42:52 docsrs cratesfyi[8350]:    5: docs_rs::utils::daemon::start_daemon
Oct 30 15:42:52 docsrs cratesfyi[8350]:    6: cratesfyi::CommandLine::handle_args
Oct 30 15:42:52 docsrs cratesfyi[8350]:    7: cratesfyi::main
Oct 30 15:42:52 docsrs cratesfyi[8350]:    8: std::rt::lang_start::{{closure}}
Oct 30 15:42:52 docsrs cratesfyi[8350]:    9: std::rt::lang_start_internal::{{closure}}
Oct 30 15:42:52 docsrs cratesfyi[8350]:              at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/rt.rs:52
Oct 30 15:42:52 docsrs cratesfyi[8350]:       std::panicking::try::do_call
Oct 30 15:42:52 docsrs cratesfyi[8350]:              at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:297
Oct 30 15:42:52 docsrs cratesfyi[8350]:       std::panicking::try
Oct 30 15:42:52 docsrs cratesfyi[8350]:              at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panicking.rs:274
Oct 30 15:42:52 docsrs cratesfyi[8350]:       std::panic::catch_unwind
Oct 30 15:42:52 docsrs cratesfyi[8350]:              at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/panic.rs:394
Oct 30 15:42:52 docsrs cratesfyi[8350]:       std::rt::lang_start_internal
Oct 30 15:42:52 docsrs cratesfyi[8350]:              at /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/libstd/rt.rs:51
Oct 30 15:42:52 docsrs cratesfyi[8350]:   10: main

cc @l4l

[jyn514]

This revert has already been deployed in production.

[jyn514]

@Nemo157 and I debugged this: you can replicate the failure locally with git remote set-url origin https://github.com/rust-lang/crates.io-index.git and running cargo run daemon. The issue was the crates-index-diff update, not the code changes.