Remove PushResult parser and logging
The plan for #2451 (closed) was to use !1845 (merged) to parse the output of a failed push mirror, remove refs that had diverged, and push again with only the would-be successful ones.
Instead, in !1915 (merged) we avoided the would-be failures altogether, and so the work done in !1845 (merged) is largely unnecessary and should be removed.
Closes #2687 (closed).