rust: kbuild: remove unneeded old `allow`s for generated layout tests

rust: kbuild: remove unneeded old `allow`s for generated layout tests

The issue that required `allow`s for `cfg(test)` code generated by
`bindgen` for layout testing was fixed back in `bindgen` 0.60.0 [1],
so it could have been removed even before the version bump, but it does
not hurt.

Thus remove it now.

Link: https://github.com/rust-lang/rust-bindgen/pull/2203
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-4-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>