Remove delta island feature flag

Delta islands were implemented last released in:
https://gitlab.com/gitlab-org/gitaly/merge_requests/1110. It's been
enabled on production and works as expected.

changelogs/unreleased/zj-remove-delta-island-feature-flag.yml

+merge_request: 28871
+title: Improve clone performance by using delta islands
+type: performance

lib/gitlab/gitaly_client.rb

@@ -33,7 +33,7 @@ module Gitlab
     SERVER_FEATURE_CATFILE_CACHE = 'catfile-cache'.freeze
     # Server feature flags should use '_' to separate words.
-    SERVER_FEATURE_FLAGS = [SERVER_FEATURE_CATFILE_CACHE, 'delta_islands'].freeze
+    SERVER_FEATURE_FLAGS = [SERVER_FEATURE_CATFILE_CACHE].freeze
     MUTEX = Mutex.new