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/memory.c

@@ -882,7 +882,6 @@ void free_all_mem(void)
 
   decor_free_all_mem();
   drawline_free_all_mem();
-  input_free_all_mem();
 
   if (ui_client_channel_id) {
     ui_client_free_all_mem();