chore: Remove SENTRY_USE_CUSTOMER_DOMAINS (#77872)

This setting is no longer used. Use `system:multi-region` feature flag
instead.

src/sentry/conf/server.py

@@ -2184,10 +2184,6 @@ SENTRY_USE_ISSUE_OCCURRENCE = False
 # This flag activates consuming GroupAttribute messages in the development environment
 SENTRY_USE_GROUP_ATTRIBUTES = True
 
-# This flag activates code paths that are specific for customer domains
-# Deprecated: This setting will be replaced with feature checks for system:multi-region
-SENTRY_USE_CUSTOMER_DOMAINS = False
-
 # This flag activates replay analyzer service in the development environment
 SENTRY_USE_REPLAY_ANALYZER_SERVICE = False