Rollup merge of rust-lang#67230 - chansuke:remove-irrelevant-passage, r=Dylan-DPC

Remove irelevant comment on `register_dtor`

Fixes rust-lang#66572.

Author: JohnTitor

src/libstd/sys/unix/fast_thread_local.rs

@@ -8,8 +8,6 @@
 // Note, however, that we run on lots older linuxes, as well as cross
 // compiling from a newer linux to an older linux, so we also have a
 // fallback implementation to use as well.
-//
-// Due to rust-lang/rust#18804, make sure this is not generic!
 #[cfg(any(
     target_os = "linux",
     target_os = "fuchsia",