refactor(input): don't use a ring for input

Since paste data is handled via a separate channel, the data processed via `input_buffer` is typically just explicit keys as typed in by the user. Therefore it should be fine to use `memmove()` to always put the remaining data in front when refilling the buffer.

src/nvim/main.c

@@ -154,7 +154,6 @@ void event_init(void)
   loop_init(&main_loop, NULL);
   resize_events = multiqueue_new_child(main_loop.events);
 
-  input_init();
   signal_init();
   // mspgack-rpc initialization
   channel_init();