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

spec/options/async_zpty_name_spec.rb

@@ -1,19 +0,0 @@
-context 'when async suggestions are enabled' do
-  let(:options) { ["ZSH_AUTOSUGGEST_USE_ASYNC="] }
-
-  describe 'the zpty for async suggestions' do
-    it 'is created with the default name' do
-      session.run_command('zpty -t zsh_autosuggest_pty &>/dev/null; echo $?')
-      wait_for { session.content }.to end_with("\n0")
-    end
-
-    context 'when ZSH_AUTOSUGGEST_ASYNC_PTY_NAME is set' do
-      let(:options) { super() + ['ZSH_AUTOSUGGEST_ASYNC_PTY_NAME=foo_pty'] }
-
-      it 'is created with the specified name' do
-        session.run_command('zpty -t foo_pty &>/dev/null; echo $?')
-        wait_for { session.content }.to end_with("\n0")
-      end
-    end
-  end
-end