x86/microcode: Mop up early loading leftovers

Get rid of the initrd_gone hack which was required to keep
find_microcode_in_initrd() functional after init.

As find_microcode_in_initrd() is now only used during init, mark it
accordingly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231017211723.298854846@linutronix.de