Get ride of the useless pubdate tag.

Get ride of the useless pubdate tag.
We are not using it, no body is using it and the information is easily
available in git.

For the record, it has been deleted with the following code:
------

function removepubdate(fileName)

doc = xmlRead(fileName);

xp = xmlXPath(doc, "//foo:info",["foo" "http://docbook.org/ns/docbook"])
xmlRemove(xp);

xmlWrite(doc);

xmlDelete(doc);
endfunction

listHelp=mgetl("helpfile.txt");

for i = 1 : size(listHelp,'*')
removepubdate(listHelp( i));
end
------

Change-Id: Id378c26518b4201373ecfc030411a6cdc7b69e22