REF: GroupBy.ops don't depend on mutation status
`self.mutated` is only used in one place (see one-line edit in first commit) and removing that doesn't break anything.  Conceivably could change some behavior but until we have an example I don't think it is even worth documenting as an API change.

Might be leftover from the old libreduction path?

cc @jreback for the deep cut into the history
Thanks @jbrockmendel 