Revert "context: Add a context stack API"

This reverts commit cab6c35e426cc8e7ef58c6b2e875c2b24f88c801.

This breaks tests (well, actually, our test suite is broken because
it creates more than one JSRuntime on the same thread, but that's
somewhat unavoidable), and we're going to look at reengineering
the coverage tool to not use multiple contexts.