[DOCS] Remove unnecessary image from Slack connector (elastic#125715) (elastic#125859)

(cherry picked from commit 08b7d18)

Co-authored-by: Lisa Cawley <[email protected]>

Author: kibanamachine

docs/user/alerting/action-types/slack.asciidoc

@@ -56,5 +56,3 @@ URL, set up an an **Incoming Webhook Integration** through the Slack console:
 image::images/slack-add-webhook-integration.png[]
 . Click *Add Incoming Webhook Integration*.
 . Copy the generated webhook URL so you can paste it into your Slack connector form.
-+
-image::images/slack-copy-webhook-url.png[]