Remove legacy VSP ticket purchasing

[jholdstock]

All legacy VSPs are closed and no longer accept new tickets purchased from Decrediton, therefore the legacy VSP ticket options can be removed.

• Remove the form to purchase legacy VSP tickets
• Check through the UI to remove any other references to legacy VSPs
• Ensure legacy tickets are still tracked correctly in ticket history lists
• Ensure decrediton is no longer calling dcrwebapi endpoint https://api.decred.org/?c=gsd

[xaur]

If someone forks dcrstakepool and patches it to work with all the new stuff, having a way to activate legacy ticket purchasing may help a few users to avoid running custom Decrediton builds.

As of writing, there are 184 legacy tickets making 0.45% of the ticket pool and worth ~$3.6M. I think when dcrstakepool stops working it may trigger some people to team up and hire a dev to patch it, so that those who really want to keep using it could do so.

I think this is all very unlikely to happen, just wanted to mention the side effect of actually removing the feature instead of hiding it.

On the other hand, if we keep it hidden, we kind of commit to test and maintain it in a working state, which is an extra cost.