Remove security_finding_build_disable_joins feature flag

Changelog: removed

config/feature_flags/development/security_finding_build_disable_joins.yml

----
-name: security_finding_build_disable_joins
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72738
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/342380
-milestone: '14.4'
-type: development
-group: group::threat insights
-default_enabled: true

ee/app/models/security/finding.rb

@@ -18,7 +18,7 @@ module Security
     belongs_to :scan, inverse_of: :findings, optional: false
     belongs_to :scanner, class_name: 'Vulnerabilities::Scanner', inverse_of: :security_findings, optional: false
 
-    has_one :build, through: :scan, disable_joins: -> { ::Feature.enabled?(:security_finding_build_disable_joins, default_enabled: :yaml) }
+    has_one :build, through: :scan, disable_joins: true
 
     enum confidence: ::Enums::Vulnerability.confidence_levels, _prefix: :confidence
     enum severity: ::Enums::Vulnerability.severity_levels, _prefix: :severity