html/tidy: rm'd unkown attribute rule

This seems like this may be specific to a particular environment/framework/toolkit, and if so, it should probably be removed. I suggest leaving this pull request open for a while and see if @Thanatermesis (the origin of this line) or anybody else has a reason for this to stay.

more info: https://github.com/scrooloose/syntastic/commit/a956a814323225f9fce873a14a3d8edc027b7aa5#commitcomment-4537304