perf: Disable clipboard in do_cmdline().

For any script--not just `:global` commands--there is no reason to
update the system clipboard until the script is finished, so disable it
during do_cmdline().

Before this change, 'clipboard=unnamedplus' causes scripted editing to
be extremely slow (e.g. `:normal` in a while-loop).

Closes #3534

src/nvim/ex_cmds2.c

@@ -1983,8 +1983,6 @@ void ex_listdo(exarg_T *eap)
     save_ei = au_event_disable(",Syntax");
   }
 
-  start_batch_changes();
-
   if (eap->cmdidx == CMD_windo
       || eap->cmdidx == CMD_tabdo
       || P_HID(curbuf)
@@ -2181,7 +2179,6 @@ void ex_listdo(exarg_T *eap)
     apply_autocmds(EVENT_SYNTAX, curbuf->b_p_syn,
                    curbuf->b_fname, true, curbuf);
   }
-  end_batch_changes();
 }
 
 /// Add files[count] to the arglist of the current window after arg "after".