removes unused context binding from Discourse.Lightbox

This is not covered by any unit tests, but the whole method is 3 lines and it is very clear that _this (nor this) is not used anywhere. Unit tests to cover this method would require quite complex setup (method loads external script asynchronously then binds it as jQuery plugin) so I think removing this line without test is a reasonable tradeoff - but it's your call if you want to accept such pull request.