ext4: avoid ext4_error when mounting a fs with a single bg

Remove some completely unneeded code which which caused an ext4_error
to be generated when mounting a file system with only a single block
group.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org