MAINTAINERS: Remove dead path from LOCKING PRIMITIVES entry

MAINTAINERS: Remove dead path from LOCKING PRIMITIVES entry

Since a622309ac87f ("locking/mutex: Kill arch specific code"), there
are no mutex header files under arch/, so we can remove the redundant
entry from MAINTAINERS.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Jason Low <jason.low2@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181001142856.GC9716@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>