Nim: Remove unused declaration

The implementation is gone for a while.

Change-Id: I79e79569b24b7af5ca5e3df0a245fa5dd0f8ef30
Reviewed-by: hjk <hjk@qt.io>

src/plugins/nim/project/nimbuildsystem.h

@@ -93,8 +93,6 @@ protected:
     void loadSettings();
     void saveSettings();
 
-    void collectProjectFiles();
-
     ParseGuard m_guard;
     NimProjectScanner m_projectScanner;
 };