Use UNLINK to remove old repository set caches

When we write a repository set cache, we first remove the old key.
Repository set cache values (tag and branch names) can be very large, so
using UNLINK instead of DEL should be more efficient.

lib/gitlab/repository_set_cache.rb

@@ -22,7 +22,7 @@ module Gitlab
 
       with do |redis|
         redis.multi do
-          redis.del(full_key)
+          redis.unlink(full_key)
 
           # Splitting into groups of 1000 prevents us from creating a too-long
           # Redis command