fs/gfs2: remove unused IS_DINODE and IS_LEAF macros

Since commit 1579343a ("GFS2: Remove dirent_first() function"),
these macros aren't used any more, so remove them.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>