Revert async process substitution & completion strategy

They're not quite ready. Keep them on a feature branch for now.

spec/integrations/client_zpty_spec.rb

@@ -1,14 +1,10 @@
 describe 'a running zpty command' do
   let(:before_sourcing) { -> { session.run_command('zmodload zsh/zpty && zpty -b kitty cat') } }
 
-  context 'when using `completion` strategy' do
-    let(:options) { ["ZSH_AUTOSUGGEST_STRATEGY=completion"] }
+  it 'is not affected by running zsh-autosuggestions' do
+    sleep 1 # Give a little time for precmd hooks to run
+    session.run_command('zpty -t kitty; echo $?')
 
-    it 'is not affected' do
-      session.send_keys('a').send_keys('C-h')
-      session.run_command('zpty -t kitty; echo $?')
-
-      wait_for { session.content }.to end_with("\n0")
-    end
+    wait_for { session.content }.to end_with("\n0")
   end
 end