Remove `deprecated_safe` and its corresponding feature gate
This unimplements the feature gate for tracking issue https://github.com/rust-lang/rust/issues/94978
The author of that tracking issue and the MCP seem to no longer exist
Afaik we can just do this, but let me know if I'm wrong
Because this feature has been idle for 3+ years, and it is in the way to finish the attribute refactoring (https://github.com/rust-lang/rust/issues/131229#issuecomment-2971351163)
When someone wants to do work on it the feature gate can be re-added
r? @jdonszelmann
1944af691e