Remove unnecessary cop disable comments

There are some left overs from the previous change of
Migration/DropTable cop so this commit removes them.

db/migrate/20200522205606_create_group_deploy_keys_group.rb

@@ -19,9 +19,7 @@ class CreateGroupDeployKeysGroup < ActiveRecord::Migration[6.0]
 
   def down
     with_lock_retries do
-      # rubocop:disable Migration/DropTable
       drop_table :group_deploy_keys_groups
-      # rubocop:enable Migration/DropTable
     end
   end
 end

db/migrate/20200610130002_create_vulnerability_statistics.rb

@@ -24,7 +24,7 @@ class CreateVulnerabilityStatistics < ActiveRecord::Migration[6.0]
 
   def down
     with_lock_retries do
-      drop_table :vulnerability_statistics # rubocop:disable Migration/DropTable
+      drop_table :vulnerability_statistics
     end
   end
 end

db/migrate/20200617150041_create_namespace_limits.rb

@@ -17,6 +17,6 @@ class CreateNamespaceLimits < ActiveRecord::Migration[6.0]
   end
 
   def down
-    drop_table :namespace_limits # rubocop:disable Migration/DropTable
+    drop_table :namespace_limits
   end
 end

db/migrate/20200622103836_create_snippet_statistics.rb

@@ -18,7 +18,7 @@ class CreateSnippetStatistics < ActiveRecord::Migration[6.0]
 
   def down
     with_lock_retries do
-      drop_table :snippet_statistics # rubocop:disable Migration/DropTable
+      drop_table :snippet_statistics
     end
   end
 end