Try changing this:
...contentWindow.document.body.getElementById(elementID)...
to this:
...contentWindow.document.getElementById(elementID)...
Edit from comments: It's not removing that element because that's not how you remove elements. Try this:
var iframe = document.getElementById('actContentToGet');
var frameDoc = iframe.contentDocument || iframe.contentWindow.document;
var el = frameDoc.getElementById(elementID);
el.parentNode.removeChild(el);
See the documentation here.
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