Remove redundant tests for isSubmapOf

[sjakobi]

As pointed out by @treeowl in #517 (comment).

[treeowl]

Redundant tests don't all need to go; they can be good intuition checks. But the ones using ==> with not-particularly-common antecedents are particularly likely to be expensive, as I understand it, because many potential test cases may be discarded. That said, if they're keeping us from running deep and broad enough tests, we can get rid of them all.