MAINTAINERS: remove arch/arm/common/time-acorn.c

This file was moved to arch/arm/mach-rpc/time.c by commit a1be5d64
("ARM: riscpc: move time-acorn.c to mach-rpc"), and the pattern for
arch/arm/mach-rpc/ already exists.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>