fix(config): Remove deprecated slack.legacy-app config from defaults (#886)

Author: BYK

sentry/config.example.yml

@@ -100,7 +100,5 @@ transaction-events.force-disable-internal-project: true
 # slack.client-id: <'client id'>
 # slack.client-secret: <client secret>
 # slack.signing-secret: <signing secret>
-## If you made your slack bot before july 2020 set legacy-app to True
-slack.legacy-app: False
 ## If legacy-app is True use verfication-token instead of signing-secret
 # slack.verification-token: <verification token>