doc: Drop no longer relevant comment

The comment was introduced in 4cf3411056f6a59fc5fe07784b6b6a512d76b046,
and since 7e4bd19785ff9120b7242ff9f15231868aaf7db4 it has been no longer
relevant.

src/qt/test/test_main.cpp

@@ -79,8 +79,6 @@ int main(int argc, char* argv[])
         setenv("QT_QPA_PLATFORM", "minimal", 0 /* overwrite */);
     #endif
 
-    // Don't remove this, it's needed to access
-    // QApplication:: and QCoreApplication:: in the tests
     BitcoinApplication app;
     app.setApplicationName("Bitcoin-Qt-test");
     app.createNode(*init);