Remove redundant mut

[0x7CFE]

This PR removes unneeded mut specifiers across the codebase.

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[0x7CFE]

In my case the only warnings left are:

warning: variable does not need to be mutable
   --> parity/target/release/build/ethcore-ipc-hypervisor-c1a2466723855f8b/out/service.rs.in:764:42
    |
764 |         modules.get_mut(&module_id).map(|mut module|
    |                                          ^^^^^^^^^^
    |
    = note: #[warn(unused_mut)] on by default

warning: variable does not need to be mutable
   --> parity/target/release/build/ethcore-ipc-hypervisor-c1a2466723855f8b/out/service.rs.in:775:42
    |
775 |         modules.get_mut(&module_id).map(|mut module|
    |                                          ^^^^^^^^^^

If I understand correctly, these sources are generated automatically. What file should I edit to remove these warnings as well?

[debris]

I believe it's ipc/hypervisor/src/service.rs.in

[0x7CFE]

Oops, I was submitting final commit 730b80b when you merged the PR :) Should I open another PR for that one?

[0x7CFE]

Final change in #6775