Clean up parser test's InterwikiLoadPrefix hook

Clean up parser test's InterwikiLoadPrefix hook

Change I36865e38 adjusted the parser test class to hook
InterwikiLoadPrefix, and prevent any other uses of that hook. Which is
ok, except it doesn't clean up after itself so it winds up breaking any
other parser tests that use the same hook.

Change-Id: I351a56ac39a44721d427e9c980eaf5fff246fb57