Clean up some uses of String::substr

Cases where the end position is either equvalent to the default or past
the end of the string.

modules/gdscript/gdscript_editor.cpp

@@ -3630,7 +3630,7 @@ void GDScriptLanguage::auto_indent_code(String &p_code, int p_from_line, int p_t
 			}
 		}
 
-		String st = l.substr(tc, l.length()).strip_edges();
+		String st = l.substr(tc).strip_edges();
 		if (st.is_empty() || st.begins_with("#")) {
 			continue; //ignore!
 		}