Remove unused DOMImplementationRegistryTestCase

As part of the initiative to get rid of ignored tests in our testsuite, DOMImplementationRegistryTestCase was identified as a candidate for removal.
There is WFLY-4416 reporting a bug and that is why the testcase was introduced but it was always ignored and there is no progress in years and no plan for resolving WFLY-4416 in foreseeable future.
For this reason we should remove this testcase. This jira will be linked to original feature request to be able to recover the test from git history in case there is a reason for it.