gpu: nova-core: bindings: remove unneeded `cfg_attr`

gpu: nova-core: bindings: remove unneeded `cfg_attr`

These were likely copied from the `bindings` and `uapi` crates, but are
unneeded since there are no `cfg(test)`s in the bindings.

In addition, the issue that triggered the addition in those crates
originally is also fixed in `bindgen` (please see the previous commit).

Thus remove them.

Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>