debugfs: remove misplaced "MMP is unsupported" message

This message was inadvertently copied from the version of do_dump_mmp
that is used when CONFIG_MMP is disabled.

Signed-off-by: Tyson Nottingham <tgnottingham@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>