Gardening: Remove [Plugin] label when no longer applicable

In https://github.com/Automattic/jetpack/pull/18608 a test plugin that was in `projects/plugins/` was accidently committed and pushed. This created a new label via the Gardening action (cool), but was removed later. The end result was a PR that didn't touch any plugin.

The label remained which ended up throwing an error for the milestone action since there was no milestone for that plugin (since it wasn't supposed to be a plugin).

Could/should we remove the Plugin label when there is no longer any touched code?

cc: @jeherve

Review the Gardening action and milestone action referenced in the issue, starting with the behavior that creates and retains the Plugin label. Trace how touched plugin code is determined and how the label is consumed; done means a PR with no remaining plugin code no longer retains that label or causes a milestone error.

Written by the indexing model from the issue text.