Hi,
After deleting Pool B (using delete Pools B command) all the disks in enclosure 2 and 3 will appear in available state. You could shut down storage controller A (using the command shutdown A), remove the SAS back end cabling between MSA controller A and disk enclosures in controller A path, restart storage controller A (using the command restart sc A), shut down storage controller B (using the command shutdown B), remove the SAS back end cabling between MSA controller B and disk enclosures in controller B path and restart storage controller B (using the command restart sc B).
Power to disk enclosure could be removed and it could be removed from the rack.
MSA 2040 would work in single controller mode.
However, I would advise against removing controller B as array will not have controller level redundancy.
MSA 2040 is an end of support life hardware.
Controller B will kick in when controller A crashes or fails completely.
Controller B will also be useful when all paths from controller A to host servers fails.