Remove uses of `incompatible_use_toolchain_transition`.

Now that Bazel 7.0 has been released, it's time to remove this tech debt.

This has been a no-op since Bazel 5.0, I've been waiting to remove the code for
two years, it's time.

Part of https://github.com/bazelbuild/bazel/issues/14127.

PiperOrigin-RevId: 592219242

protos/bazel/upb_cc_proto_library.bzl

@@ -267,7 +267,6 @@ _upb_cc_proto_library_aspect = aspect(
     attr_aspects = ["deps"],
     fragments = ["cpp"],
     toolchains = use_cpp_toolchain(),
-    incompatible_use_toolchain_transition = True,
 )
 
 upb_cc_proto_library = rule(