XCB No longer lists ThreadedOpenGL capability

Even with the more recent libxcb, it's a bit flaky. Just don't list
having this capability until it is more stable.

Change-Id: I1c2b39d757803fc23295944bc9a00e39ff4e1bbf
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -204,12 +204,7 @@ bool QXcbIntegration::hasCapability(QPlatformIntegration::Capability cap) const
 
     case ThreadedPixmaps: return true;
     case OpenGL: return true;
-    case ThreadedOpenGL:
-#ifdef XCB_POLL_FOR_QUEUED_EVENT
-        return true;
-#else
-        return false;
-#endif
+    case ThreadedOpenGL: return false;
     default: return QPlatformIntegration::hasCapability(cap);
     }
 }