Remove ci_runners_tokens_optional_encryption ff

This feature flag was introduced in 11.6 and is now ready to be removed.

Changelog: removed

app/models/ci/runner.rb

@@ -13,7 +13,7 @@ class Runner < ApplicationRecord
     include Gitlab::Utils::StrongMemoize
     include TaggableQueries
-    add_authentication_token_field :token, encrypted: -> { Feature.enabled?(:ci_runners_tokens_optional_encryption, default_enabled: true) ? :optional : :required }
+    add_authentication_token_field :token, encrypted: :optional
     enum access_level: {
       not_protected: 0,

config/feature_flags/development/ci_runners_tokens_optional_encryption.yml

----
-name: ci_runners_tokens_optional_encryption
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/8638
-rollout_issue_url: 
-milestone: '11.6'
-type: development
-group: group::pipeline execution
-default_enabled: true