Remove rustc_version_check from build.rs

[FreezyLemon]

This was added in #1552 but is no longer necessary. The cfg value nightly_rustc does not seem to be used anywhere, and rust-version in Cargo.toml allows a built-in way of checking the MSRV.

rust-version is only supported in toolchains >=1.56, but all older toolchains will fail compilation for other reasons (mostly edition = "2021").

Examples:

$ cargo +1.55 c
error: failed to parse manifest at `/.../Development/rav1e/Cargo.toml`

Caused by:
  feature `edition2021` is required

$ cargo +1.60 c
error: package `clap_complete v4.4.5` cannot be built because it requires rustc 1.70.0 or newer, while the currently active rustc version is 1.60.0

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7d455e8) 88.48% compared to head (b31b792) 88.48%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3327 +/- ## ======================================= Coverage 88.48% 88.48% ======================================= Files 88 88 Lines 28230 28230 ======================================= Hits 24980 24980 Misses 3250 3250 ```

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[lu-zero]

Thank you!