ext4: Remove unused extern variables declaration

ext4_mb_stats & ext4_mb_max_to_scan are never used. We use
sbi->s_mb_stats and sbi->s_mb_max_to_scan instead.
Hence kill these extern declarations.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/928b3142062172533b6d1b5a94de94700590fef3.1685449706.git.ojaswin@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>