ocfs2: clean up unused local variables in ocfs2_file_write_iter

Since commit 86b9c6f3

 ("ocfs2: remove filesize checks for sync I/O
journal commit") removes filesize checks for sync I/O journal commit,
variables old_size and old_clusters are not actually used any more.  So
clean them up.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>