Revert "block: introduce the BFQ-v6 I/O sched for 3.0"

This reverts commit fb71900fb8fc584c2afd8cb2df365606c4b5d4f7.
Revert "block: prepare I/O context code for BFQ-v6 for 3.0"

This reverts commit decdc8677be26f0c8d42e921bc7b124eede5b3e1.