cfq-iosched: Delete unused function min_vdisktime()

This fixes the following warning when building with clang:

    block/cfq-iosched.c:970:19: error: unused function 'min_vdisktime'
        [-Werror,-Wunused-function]
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/cfq-iosched.c

@@ -978,15 +978,6 @@ static inline u64 max_vdisktime(u64 min_vdisktime, u64 vdisktime)
 	return min_vdisktime;
 }
 
-static inline u64 min_vdisktime(u64 min_vdisktime, u64 vdisktime)
-{
-	s64 delta = (s64)(vdisktime - min_vdisktime);
-	if (delta < 0)
-		min_vdisktime = vdisktime;
-
-	return min_vdisktime;
-}
-
 static void update_min_vdisktime(struct cfq_rb_root *st)
 {
 	struct cfq_group *cfqg;