remove special properties of `caskroom_only`

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
Merge pull request #4866 from rolandwalker/demote_caskroom_only

DSL: remove special properties of `caskroom_only` stanza