Revert #886 and refactor (#900)

* Revert "make completion popup windows have bufhidden=wipe (#886)"

This reverts commit f573479528.

* Remove redundant nvim_buf_set_option

`buftype=nofile` and `bufhidden=hide` are already set by
`nvim_create_buf(false, true)`.

lua/cmp/utils/buffer.lua

@@ -20,8 +20,6 @@ buffer.ensure = function(name)
   if not buf then
     created_new = true
     buf = vim.api.nvim_create_buf(false, true)
-    vim.api.nvim_buf_set_option(buf, 'buftype', 'nofile')
-    vim.api.nvim_buf_set_option(buf, 'bufhidden', 'wipe')
     buffer.cache[name] = buf
   end
   return buf, created_new