MAINTAINERS: remove firmware/isci/

This directory was removed by commit 7d99b3ab ("isci, firmware:
Remove isci fallback parameter blob and generator").

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Cc: Intel SCU Linux support <intel-linux-scu@intel.com>
Cc: Lukasz Dorau <lukasz.dorau@intel.com>
Cc: Maciej Patelczyk <maciej.patelczyk@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>