ext4: remove extra define of ext4_new_blocks_old from mballoc.c

The function prototype of ext4_new_blocks_old() is defined in ext4_fs.h,
so we don't need the extra function prototype in mballoc.c

Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>