ext4: remove unused enum EXT4_FC_COMMIT_FAILED

Below commit removed all references of EXT4_FC_COMMIT_FAILED.
commit 0915e464

 ("ext4: simplify updating of fast commit stats")

Just remove it since it is not used anymore.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/c941357e476be07a1138c7319ca5faab7fb80fc6.1647057583.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>