remove special properties of caskroom_only

HOLD until Homebrew#4865 is merged.

After Homebrew#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 Homebrew#4865 and subsequent cleanup on Casks that
use this form.

References: Homebrew#4688

lib/cask/artifact/caskroom_only.rb

@@ -1,4 +1,4 @@
-class Cask::Artifact::CaskroomOnly < Cask::Artifact::Pkg
+class Cask::Artifact::CaskroomOnly < Cask::Artifact::Base
   def self.artifact_dsl_key
     :caskroom_only
   end