Remove unused Block#end_tag method.

@Shopify/liquid for review

There is an empty end_tag method defined on the Block class.  I assume the purpose is for extensibility, although I don't see any uses of it in Shopify or Jekyll.  However, the same thing can be done by overriding the parse method with `super` followed by whatever was previously in the `end_tag` method.

I think we should consider removing the `end_tag` method to simplify the code.  Since we already have a liquid 3 release candidate, I am fine with this waiting until we start working on liquid 4.