cam/da: Remove redundant bp assignment

[imp]

Summary

We assigned bp before we fell into this loop. Since nothing can change
the bp associated with a ccb after we've assigned it in the da driver,
there's no need for this assignment.

Sponsored by: Netflix

[imp]

When the selected delete method isn't supported, and we retry with others, we can make bp go NULL since we requeue the command so it's reprocessed in dastart again. different delete methods have different limitations on size, so we can't just transform it to something else or fail the I/O.