Remove AppVeyor VS2015 build

Our VS2015 AppVeyor has been failing and we decided to remove it.

I'll make an attempt at a patch.

added CI label

mentioned in merge request !117 (closed)

mentioned in merge request !122 (merged)

mentioned in merge request !123 (merged)

Made the appropriate PRs, and opened #40096 (closed) for the 32-bit AppVeyor builds as discussed in the failed !117 (closed). Let me know if I messed something up.

LGTM; all is merged now.

closed

added BugSmashFund label