stage: rename force argument to unpersist

[ghost]

In stage.remove_outs there's a parameter force that is used to remove "persistent outputs":

    def remove_outs(self, ignore_remove=False, force=False):
        """Used mainly for `dvc remove --outs` and :func:`Stage.reproduce`."""
        for out in self.outs:
            if out.persist and not force:
                out.unprotect()
            else:
                logger.debug(
                    "Removing output '{out}' of '{stage}'.".format(
                        out=out, stage=self.relpath
                    )
                )
                out.remove(ignore_remove=ignore_remove)

I think unpersist makes more sense in this case.