Failing to remove all of the rules which were added causes action at a
distance with other tests. The two methods were also only used by
test code, making their existence in zerver.lib.rate_limiter clearly
misplaced.
This fixes one instance of a mis-balanced add/remove, which caused
tests to start failing if run non-parallel and one more anonymous
request was added within a rate-limit-enabled block.
(cherry picked from commit 0dbe111ab3)