Update my maintenance areas in MAINTAINERS file

I've dropped maintenance of the mep target.  Additionally, I'm removed
myself as an authorized committer for PowerPC, ia64, AIX, and
GNU/Linux PPC native.

gdb/MAINTAINERS

diff --git a/gdb/MAINTAINERS b/gdb/MAINTAINERS
index 927be43..187a1f6 100644
--- a/gdb/MAINTAINERS
+++ b/gdb/MAINTAINERS
@@ -366,7 +366,6 @@
 	mcore		Deleted
 
 	mep		--target=mep-elf
-			Kevin Buettner		kevinb@redhat.com
 
 	microblaze	--target=microblaze-xilinx-elf
 			--target=microblaze-linux-gnu
@@ -514,13 +513,9 @@
 Blackfin		Mike Frysinger		vapier@gentoo.org
 CRIS			Hans-Peter Nilsson	hp@axis.com
 IA64			Jeff Johnston		jjohnstn@redhat.com
-PowerPC			Kevin Buettner		kevinb@redhat.com
 S390			Ulrich Weigand		uweigand@de.ibm.com
 djgpp			DJ Delorie		dj@delorie.com
 			[Please use this address to contact DJ about DJGPP]
-ia64			Kevin Buettner		kevinb@redhat.com
-AIX			Kevin Buettner		kevinb@redhat.com
-GNU/Linux PPC native	Kevin Buettner		kevinb@redhat.com
 Pascal support		Pierre Muller		muller@sourceware.org