MAINTAINERS: iomap: Remove fs/iomap.c record

Update MAINTAINERS to reflect that fs/iomap.c file
was splitted into separate files in fs/iomap/

Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: linux-fsdevel@vger.kernel.org
Fixes: cb7181ff

 ("iomap: move the main iteration code into a separate file")
Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>