Project type term argument validation shouldn't ask for the vocabulary to use

While I was working on #455922: Make the alphabetical list of projects not suck so much I noticed some weirdness with project_plugin_argument_validate_project_type_term. It's needlessly exposing the vocab selection in the arg validation UI. This is bad for two reasons:

1) It makes the UI more complicated/busy than it needs to be
2) It makes the resulting exported views less portable, since the vid gets hard-coded in the view

We can just use _project_get_vid() directly instead of asking the user about this.