fix: Remove unused CUSTOM_RULE_START import and reference

The constant CUSTOM_RULE_START was removed from sentry.models.dynamicsampling
but the import and usage in dynamic_sampling/rules/utils.py was not cleaned up,
causing a runtime ImportError.

Author: cursoragent

src/sentry/dynamic_sampling/rules/utils.py

@@ -6,7 +6,6 @@
 from django.conf import settings
 from redis import StrictRedis
 
-from sentry.models.dynamicsampling import CUSTOM_RULE_START
 from sentry.relay.types import RuleCondition
 from sentry.utils import redis
 
@@ -73,7 +72,6 @@ class RuleType(Enum):
     RuleType.MINIMUM_SAMPLE_RATE_RULE: 1006,
     RuleType.BOOST_LOW_VOLUME_TRANSACTIONS_RULE: 1400,
     RuleType.BOOST_LATEST_RELEASES_RULE: 1500,
-    RuleType.CUSTOM_RULE: CUSTOM_RULE_START,
 }
 REVERSE_RESERVED_IDS = {value: key for key, value in RESERVED_IDS.items()}