Enable a spec for Warning.warn that is triggered by duplicated Hash literal keys

Author: andrykonchin

spec/tags/core/warning/warn_tags.txt

@@ -2,4 +2,3 @@ slow:Warning.warn has Warning as the method owner
 slow:Warning.warn can be overridden
 slow:Warning.warn does not add a newline
 slow:Warning.warn returns nil
-fails(https://github.com/ruby/prism/issues/2005):Warning.warn is called by parser warnings