ext4: remove #ifdef CONFIG_EXT4_INDEX

CONFIG_EXT4_INDEX is not an exposed config option in the kernel, and it is
unconditionally defined in ext4_fs.h.  tune2fs is already able to turn off
dir indexing, so at this point it's just cluttering up the code.  Remove
it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>