Remove detected internal links metadata additions from the cache
Remove detected internal links metadata additions from the cache

* This makes sure metadata for failures to be re-checked are not duplicated and do not pile up across consecutive runs, fixing what seen e.g. in #733.
* We also explicitly ignore the URLs for which no metadata addition is detected.
* This fixes the newly-extended test revealing the issues.

gjtorikian/html-proofer,gjtorikian/html-proofer,gjtorikian/html-proofer,gjtorikian/html-proofer,gjtorikian/html-proofer