Use either the innerText or textContent property of the A element as appropriate:
var theText = aElement.innerText || aElement.textContent;
All browsers in use support one or the other, some both.
If you want to replace the content of all links with plain text, you could do something like:
var link, links = document.links;
for (var i=0, iLen=links.length; i<iLen; i++) {
link = links[i];
link.innerHTML = link.innerText || link.textContent;
}