Remove deprecated identify slack scope (#5192)

# What this PR does
Tested on dev (by manually changing the sign in URL in oauth flow). All
works correctly


## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/social_auth/backends.py

@@ -9,7 +9,7 @@
 
 # Scopes for slack user token.
 # It is main purpose - retrieve user data in SlackOAuth2V2 but we are using it in legacy code or weird Slack api cases.
-USER_SCOPE = ["channels:read", "identify", "users.profile:read", "users:read", "users:read.email"]
+USER_SCOPE = ["channels:read", "users.profile:read", "users:read", "users:read.email"]
 
 # Scopes for slack bot token.
 # It is prime token we are using for most requests to Slack api.