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

[charlespierce]

Related to #222
Closes #562
Closes #216
Closes #165
Closes #104
Closes #99

Info

• As part of the push towards automatic updates, we want to enable Volta to work directly on the command line, instead of through a shell function wrapper.
• The purpose of the function wrapper was to allow the volta activate and volta deactivate commands to work as expected, modifying the PATH environment variable for the current shell.
• However, those commands were intended as a release valve for when we were in early testing phases, and are no longer needed since Volta is much more stable now.

Changes

• Removed the activate and deactivate commands from the Volta CLI and the associated ActivityType
• Moved the VOLTA_UNSAFE_GLOBAL environment variable definition to the run module, where it is used.
• Removed the shell and env modules, which previous existed primarily to support the Postscript behavior that is no longer needed without activate / deactivate
• Removed System::enabled_path, which was only used by volta activate before.
• Deleted tests that referenced no-longer-available blocks / commands.

Tested

• Verified that volta activate and volta deactivate are no longer allowed commands
• All automated tests pass

Notes

• This PR builds on top of [volta updates] Add volta-updates feature and CI override #558, as it uses the feature flag added in that PR, so it will remain a draft until that is merged.
• Since this is connected to the automatic updates, all of the changes are locked behind the volta-updates feature flag, so "removal" generally meant adding a #[cfg(not(feature = "volta-updates"))] attribute to the relevant block / line of code.

[thoov]

@charlespierce Should we deprecate these first? I know they aren't public but might be a good practice to either version bump this to 0.7.0 or include at least one version with a warning deprecation before removing

[charlespierce]

@thoov Yeah, that's a great idea. This change is locked behind the updates feature flag and won't roll out until we're ready to have that entire feature, which will definitely be a 0.7.0 release, but I'll make another PR that adds a deprecation warning to volta activate and volta deactivate for the 0.6.x releases.

[charlespierce]

Created #571 to add the deprecation warnings to the current block

[chriskrycho]

Ship it!