x86/microcode: Remove ->request_microcode_user()

  181b6f40 ("x86/microcode: Rip out the OLD_INTERFACE")

removed the old microcode loading interface but forgot to remove the
related ->request_microcode_user() functionality which it uses.

Rip it out now too.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220825075445.28171-1-bp@alien8.de