ext4: Remove stale block allocator references from ext4.h

Remove some leftovers from when the old block allocator was removed
(c2ea3fde).  ext4_sb_info is now a bit lighter.  Also remove a dangling
read_block_bitmap() prototype.

Signed-off-by: Mike Snitzer <snitzer@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>