x86: Remove obsolete LOCK macro from include/asm-x86/atomic_64.h

Commit d167a518

 "[PATCH] x86_64: x86_64 version of the smp alternative patch."
has left the LOCK macro in include/asm-x86_64/atomic.h, which is now
include/asm-x86/atomic_64.h. Its scope should be local to the file, other
architectures don't provide it, I couldn't find an in-tree user of it and
allyesconfig, allmodconfig and allnoconfig build fine without it, so this patch
removes it.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>