Remove feature flag for using UNLINK in Redis

UNLINK was added in Redis 4. We put this usage of it behind a feature
flag in case there was an issue, and also added a fallback for instances
using Redis 3.

We no longer support Redis 3 and we haven't needed to disable the
feature flag, so we can just simplify this to always use UNLINK.

lib/gitlab/set_cache.rb

@@ -22,7 +22,7 @@ module Gitlab
         keys = keys.map { |key| cache_key(key) }
 
         Gitlab::Instrumentation::RedisClusterValidator.allow_cross_slot_commands do
-          unlink_or_delete(redis, keys)
+          redis.unlink(*keys)
         end
       end
     end
@@ -60,17 +60,5 @@ module Gitlab
     def with(&blk)
       Gitlab::Redis::Cache.with(&blk) # rubocop:disable CodeReuse/ActiveRecord
     end
-
-    def unlink_or_delete(redis, keys)
-      if Feature.enabled?(:repository_set_cache_unlink, default_enabled: true)
-        redis.unlink(*keys)
-      else
-        redis.del(*keys)
-      end
-    rescue ::Redis::CommandError => e
-      Gitlab::ErrorTracking.log_exception(e)
-
-      redis.del(*keys)
-    end
   end
 end

spec/lib/gitlab/repository_set_cache_spec.rb

@@ -93,23 +93,6 @@ RSpec.describe Gitlab::RepositorySetCache, :clean_gitlab_redis_cache do
 
       it { is_expected.to eq(0) }
     end
-
-    context "unlink isn't supported" do
-      before do
-        allow_any_instance_of(Redis).to receive(:unlink) { raise ::Redis::CommandError }
-      end
-
-      it 'still deletes the given key' do
-        expect(cache.expire(:foo)).to eq(1)
-        expect(cache.read(:foo)).to be_empty
-      end
-
-      it 'logs the failure' do
-        expect(Gitlab::ErrorTracking).to receive(:log_exception)
-
-        cache.expire(:foo)
-      end
-    end
   end
 
   describe '#exist?' do