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>