rust: kbuild: remove skipping of `-Wrustdoc::unescaped_backticks`

rust: kbuild: remove skipping of `-Wrustdoc::unescaped_backticks`

Back in Rust 1.82.0, I cleaned the `rustdoc::unescaped_backticks` lint in
upstream Rust and added tests so that hopefully it would not regress [1].

Thus we can remove it from our side given the Rust minimum version bump.

Link: https://github.com/rust-lang/rust/pull/128307
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-12-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>