MAINTAINERS: remove Stelian from the AMS driver record

This driver is PPC only, and I am no longer able to test it.

Signed-off-by: Stelian Pop <stelian@popies.net>
Cc: Michael Hanselmann <linux-kernel@hansmi.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>