CSS selector :enabled no longer matches Anchor, Area & Link elements

HTML spec has been modified [1] to disable support for :enabled CSS
selector on Anchor, Area & Link elements, after discussion on W3C
Bugzilla [2].

[1] https://html5.org/r/8818
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26622
auto merge of #3491 : brunoabinader/servo/css-selector-enabled-remove-anchor-area-link-elems, r=Ms2ger

HTML spec has been modified [1] to disable support for :enabled CSS
selector on Anchor, Area & Link elements, after discussion on W3C
Bugzilla [2].

[1] https://html5.org/r/8818
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26622

Next step: Move :enabled CSS selector content test to web-platform-tests.