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
Merge "Clean up parser test's InterwikiLoadPrefix hook"