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

This reverts commit e5dcc6ece7402eec74f2ec1a575bf35ffad7f68c.
Revert "block: prepare I/O context code for BFQ-v6 for 3.0"

This reverts commit 2c09ead5491e8a052468a8a9a3bb22a6a2dc7d3a.