ui: Reimplement :suspend command for remote UIs.

- Remove suspend method from the UI protocol
- Handle `:suspend` by disconnecting the last channel that sent a request to
  nvim.

src/nvim/ui.c

@@ -117,9 +117,7 @@ void ui_write(uint8_t *s, int len)
  */
 void ui_suspend(void)
 {
-  if (abstract_ui) {
-    UI_CALL(suspend);
-  } else {
+  if (!abstract_ui) {
     mch_suspend();
   }
 }