Remove more bits and pieces of Internet Explorer 8 support

Some of these are just comments that were explaining code removed in e843ae7a4cfb746760e1d8c3578d9ec613c3e2ee. The most relevant (functional) change is the one to ngSanitize.

I've also removed adjusted some comments referring to IE8 and IE9 to only mention IE9.
It always used textContent (the [decision point](https://github.com/angular/angular.js/pull/9478/files#diff-699beb0d8c48a8257d3d70331bf6138dL403) was removed) so there's IMO no point in testing this.