ext4: remove extra IS_RDONLY() check

ext4_change_inode_journal_flag() is only called from one location:
ext4_ioctl(EXT3_IOC_SETFLAGS).  That ioctl case already has a IS_RDONLY()
call in it so this one is superfluous.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>