remove special properties of `caskroom_only` stanza

HOLD until #4865 is merged.

After #4865, `caskroom_only` can be subclassed from `Cask::Artifact::Base`
like any other artifact.  It no longer has the special property of making
`uninstall` stanzas work.

This stanza has never been documented.  It is possible that it will no longer
be needed, after #4865 and subsequent cleanup on Casks that use this form.

References: #4688