Revert "Fix a memory leak caused by Banzai::Filter::SanitizationFilter"

This reverts commit 504a3b5e.

lib/banzai/filter/sanitization_filter.rb

@@ -7,7 +7,7 @@ module Banzai
       UNSAFE_PROTOCOLS = %w(data javascript vbscript).freeze
       def whitelist
-        whitelist = super.dup
+        whitelist = super
         customize_whitelist(whitelist)
@@ -42,8 +42,6 @@ module Banzai
         # Allow any protocol in `a` elements...
         whitelist[:protocols].delete('a')
-        whitelist[:transformers] = whitelist[:transformers].dup
-
         # ...but then remove links with unsafe protocols
         whitelist[:transformers].push(remove_unsafe_links)