x86/microcode/intel: Remove pointless mutex

There is no concurrency.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230812195728.069849788@linutronix.de