fs/ufs/super.c: remove set but not used variable 'usb3'

fs/ufs/super.c: remove set but not used variable 'usb3'

Fixes gcc '-Wunused-but-set-variable' warning:

  fs/ufs/super.c: In function ufs_statfs:
  fs/ufs/super.c:1409:32: warning: variable usb3 set but not used [-Wunused-but-set-variable]

It is not used since commmit c596961d1b4c ("ufs: fix s_size/s_dsize
users")

Link: http://lkml.kernel.org/r/20190525140654.15924-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Evgeniy Dushistov <dushistov@mail.ru>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>