x86/mm: Remove unused microcode.h include

No usage for anything in that header.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230812195727.600549655@linutronix.de