certs: remove misleading comments about GCC PR

This dependency is necessary irrespective of the mentioned GCC PR
because the embedded certificates are build artifacts and must be
generated by extract_certs before *.S files are compiled.

The comment sounds like we are hoping to remove these dependencies
someday. No, we cannot remove them.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>