ext4: cleanup never-used magic numbers from htree code

dx_root_limit() will had some dead code which forced it to always return
20, and dx_node_limit to always return 22 for debugging purposes.
Remove it.

Acked-by: Andreas Dilger <adilger@sun.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>