Cleanup: Remove some unused functions. (#11429)

These were picked up with cppcheck.

src/newgrf_text.cpp

@@ -744,8 +744,6 @@ struct TextRefStack {
 		this->grffile = grffile;
 		this->used = true;
 	}
-
-	void RewindStack() { this->position = 0; }
 };
 
 /** The stack that is used for TTDP compatible string code parsing */
@@ -819,11 +817,6 @@ void StopTextRefStackUsage()
 	_newgrf_textrefstack.used = false;
 }
 
-void RewindTextRefStack()
-{
-	_newgrf_textrefstack.RewindStack();
-}
-
 /**
  * FormatString for NewGRF specific "magic" string control codes
  * @param scc   the string control code that has been read