Don't send schedule confirmations for chat jobs

Previously whenever you'd schedule a chatops command we'd respond with
the message "The command has been scheduled!" which was a link to the
new build. Such messages can get annoying very quickly.

Fortunately there's a trick we can use: when you send back an empty
message Slack won't show said message, but it _will_ show the input
command.

ee/changelogs/unreleased/chatops-output.yml

+---
+title: Don't send schedule confirmations for chat jobs
+merge_request:
+author:
+type: changed

ee/lib/gitlab/chat/responder/slack.rb

@@ -47,9 +47,10 @@ module Gitlab
 
         # Returns the output to send back after a command has been scheduled.
         def scheduled_output
-          {
-            text: message_text("<#{build_url}|The command has been scheduled!>")
-          }
+          # We return an empty message so that Slack still shows the input
+          # command, without polluting the channel with standard "The job has
+          # been scheduled" (or similar) responses.
+          { text: '' }
         end
 
         private

ee/spec/lib/gitlab/chat/responder/slack_spec.rb

@@ -69,7 +69,7 @@ describe Gitlab::Chat::Responder::Slack do
     it 'returns the output for a scheduled build' do
       output = responder.scheduled_output
 
-      expect(output[:text]).to match(/<@U123>:.+The command has been scheduled!/)
+      expect(output).to eq({ text: '' })
     end
   end
 end