This is a little tricky, but possible. First, add a JavaScript node before the write images node. Then delete all the boilerplate code and paste the following in:
function processAsset(document, jsnode, asset) {
var s = asset.outputFileName().replace(" page 1.", ".")
asset.setOutputFileName(s);
return true;
}
That little snippet of code removes the page 1 bit, and then sets the output file name to the new string.