[Updates] Remove 'volta activate' and 'volta deactivate'

[charlespierce]

volta deactivate (and later volta activate) are commands that were added early on as an escape hatch for when Volta wasn't working right and we needed to give a quick escape for our beta users. As Volta has matured and stabilized, they have become less important.

Making these two commands work requires additional setup and for volta to be implemented as a shell function wrapper that calls out to the Volta binary. This implementation is major roadblock in supporting #222, as well as resulting in a number of other issues: #216, #165, #104, and #99.

Beyond the issues that result from the function wrappers, they also make it more difficult to install Volta in a "traditional" way by putting the binary directly on the PATH. That is an approach we want to take to support automatic updates, so we also have pressure from that direction to remove the function wrapper (and as a result, the activate and deactivate commands).

For all these reasons, the commands volta activate and volta deactivate should be deprecated and then removed.

[rwjblue]

I'd like to see this considered blocked on #292. Without a way to volta pin npm or volta deactivate, I'd have to modify my shell config in order to debug issues with specific npm versions (e.g. emberjs/data#6589).