Remove target=_blank from pages
Drop performance platform specific external link

Finishes [#64156102]

The story requires that there is no Performance Platform specific
styling for external links. There are some images and styling that is,
so these should be blatted and we should rely on the a[rel="external"]
styling using mixins from govuk_frontent_toolkit
Merge pull request #171 from alphagov/rel-external

Rel external