Remove [lints.rust] section from rustc_type_ir.

It was added in #129523 to enable building on stable when there were
`cfg(bootstrap)` occurrences in the crate. But those are gone now, so
the section can be removed.

Author: nnethercote

compiler/rustc_type_ir/Cargo.toml

@@ -33,6 +33,3 @@ nightly = [
     "rustc_index/nightly",
     "rustc_ast_ir/nightly",
 ]
-
-[lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bootstrap)'] }