Remove unnecessary Jekyll::Tags::IncludeTag#blank? method

This was introduced in 421e58ad to fix a break of backwards compatibility in Liquid 2.5 (https://github.com/Shopify/liquid/pull/218). The commit that introduced that bug was reverted (the 2.5 and 2.6 branches don't contain any blank tag optimization), so this is not an issue anymore.

Also, the "bug" was fixed on master (upcoming Liquid 3.0) in https://github.com/Shopify/liquid/commit/2efe809e11ee5db6feea5e62d6c6585471f9f8b5, so this shouldn't be necessary anymore.

@parkr, please take quick look.
cc @zerobase