test: remove FLAKY for test-domain-error-types

Doesn't seem to have failed in the last 2 or 3 days (and probably longer). It was failing in debug builds but there aren't any yellow debug builds in https://ci.nodejs.org/job/node-test-commit-linux-containered/nodes=ubuntu1804_sharedlibs_debug_x64/ so this is probably either not an issue anymore or is very rare. (If it's still a problem, we'll want to open an issue for it anyway rather than have it in default-hidden comments in a pull request that's landed.)