Remove deprecated Linker::getExternalLinkAttributes method

deprecated since 1.18 and unused anywhere else in
core or all the extensions in gerrit:

https://github.com/search?utf8=%E2%9C%93&q=getExternalLinkAttributes+%40wikimedia&type=Code&ref=searchresults

Change-Id: Id5ae9186b3a83672c8310adb5dfdaf07bfec3d74