Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

src/start.zsh

@@ -14,10 +14,6 @@ _zsh_autosuggest_start() {
 	# zsh-syntax-highlighting widgets. This also allows modifications
 	# to the widget list variables to take effect on the next precmd.
 	add-zsh-hook precmd _zsh_autosuggest_bind_widgets
-
-	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
-		_zsh_autosuggest_async_start
-	fi
 }
 
 # Start the autosuggestion widgets on the next precmd