fs/ext4/super.c: remove unused `deprecated_msg'

fs/ext4/super.c:1744:19: warning: 'deprecated_msg' defined but not used [-Wunused-const-variable=]

Reported-by: kernel test robot <lkp@intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>