x86: deduplicate the spectre_v2_user documentation

This would need updating to make prctl be the new default, but it's
simpler to delete it and refer to the dup.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201105001406.13005-2-aarcange@redhat.com