destroy signatures instead of updating them

This is faster for the deployment process, as the destroyed signatures
will be re-generated on demand again, instead of updating them all on
deploy.

db/post_migrate/20170830084744_destroy_gpg_signatures.rb

+class DestroyGpgSignatures < ActiveRecord::Migration
+  DOWNTIME = false
+
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  class GpgSignature < ActiveRecord::Base
+    self.table_name = 'gpg_signatures'
+
+    include EachBatch
+  end
+
+  def up
+    GpgSignature.each_batch do |relation|
+      relation.delete_all
+    end
+  end
+
+  def down
+  end
+end

db/post_migrate/20170830084744_update_gpg_signatures_verification_status.rb

-class UpdateGpgSignaturesVerificationStatus < ActiveRecord::Migration
-  DOWNTIME = false
-
-  def up
-    GpgSignature.where(verification_status: nil).find_each do |gpg_signature|
-      gpg_signature.gpg_commit.update_signature!(gpg_signature)
-    end
-  end
-
-  def down
-    # we can't revert setting the verification_status, but actually we don't
-    # need to really, as setting this is not a harmful change.
-  end
-end