staging: Remove LTTng from MAINTAINERS file

LTTng has been removed from the staging tree. Complete this removal by
removing the LTTng entry from the MAINTAINERS file.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Ingo Molnar <mingo@elte.hu>
CC: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>