MAINTAINERS: remove the MTRR entry

I haven't seen Richard doing MTRR related work for quite some time, and
the "X86 ARCHITECTURE" entry in MAINTAINERS already covers the people
currently responsible for this code.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>