Remove CategoryTree integration tests

Test, rather. It does more harm than good, making Cite annoying to
test in isolation (breaking on WMF jenkins setup) and doesn't really
add much value. The clearState() behavior fixed per bug 20748 should
be tested with a regular PHPUnit test, if anything.

Bug: 54126
Change-Id: I6b65ef7280f95a537f25b7f5c9e57434c21357dd
Merge "Remove CategoryTree integration tests"