"Fix" -Wself-assign-overloaded

clang has a warning about code such as "a = a". Unfortunately, there's some code in kwin that looks that way.

Regardless of where such code appears, it's wrong and it should be removed. Given that and the fact that xcbutils are used solely by kwin, this change removes relevant tests.

Another option would be to add "-Wno-self-assign-overloaded", but it will be great to avoid adding any compiler-specific quirks and "-Wself-assign-overloaded" can be actually useful for catching code smells such as "a = a" or "a -= a", etc.