MAINTAINERS: Remove self from powerpc EEH

I'm sorry to say I can no longer maintain this position.

Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aec7d729c28e35c7fa9969ec50229080c771195c.1593471043.git.sbobroff@linux.ibm.com