x86/microcode: Remove microcode_mutex.

microcode_mutex is only used by reload_store(). It has a comment saying
"to synchronize with each other". Other user of this mutex have been
removed in the commits
	181b6f40 ("x86/microcode: Rip out the OLD_INTERFACE").
	b6f86689

 ("x86/microcode: Rip out the subsys interface gunk")

The sysfs interface does not need additional synchronisation vs itself
because it is provided as kernfs_ops::mutex which is acquired in
kernfs_fop_write_iter().

Remove superfluous microcode_mutex.

Link: https://lore.kernel.org/r/20230803083253.VGMnC9Gd@linutronix.de
Link: https://lore.kernel.org/r/20230804075853.JF_n6GXC@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>