pine64-pinetab: change maintainers

My engineering prototype pinetab is close to unusable, so I can't use it to test pmOS. Also, PINE64 is no longer selling these devices, and the ones that were sent previously to some developers use different display panel anyway.

Also maybe it makes sense to remove PINE64 PineTab from community category as well, I'd better have there devices that you can actually buy and use (line Pinebook Pro).