Regression introduced in #1754 which meant that (due to storing the
current formula revision in `attributes_map`) `stable_revisions.empty?`
would never be `true`.

Fixes an issue introduced in #1192 where there would be a failure if the
alias link didn't exist on removal (which would be the case for anything
with an alias installed before #1192 was merged).

This will become particularly relevant when there’s a new version of a
formula and the existing alias needs to be removed.

Current version aliases should be provided for versioned formulae so
people can `brew install foo@1.2` to provide pin-like behaviour.

If `brew instal` is permissible, `brew uninstal` should be, too.

This doesn't actually seem to make that much difference,
but it is at least consistent with the main class body and with
the Linux overrides.