x86/microcode: Remove remaining references to CONFIG_MICROCODE_AMD

Commit e6bcfdd7

 ("x86/microcode: Hide the config knob") removed the
MICROCODE_AMD config, but left some references in defconfigs and comments,
that have no effect on any kernel build around.

Clean up those remaining config references. No functional change.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230825141226.13566-1-lukas.bulwahn@gmail.com