startuptime: always enable startuptime

Removes the STARTUPTIME define.

src/nvim/globals.h

@@ -1207,9 +1207,7 @@ EXTERN char bot_top_msg[] INIT(= N_("search hit BOTTOM, continuing at TOP"));
 /* For undo we need to know the lowest time possible. */
 EXTERN time_t starttime;
 
-#ifdef STARTUPTIME
 EXTERN FILE *time_fd INIT(= NULL);  /* where to write startup timing */
-#endif
 
 /*
  * Some compilers warn for not using a return value, but in some situations we