jquery.tablesorter: Remove dead node check in getElementSortKey()

jquery.tablesorter: Remove dead node check in getElementSortKey()

In 2012, commit 39d1f3967be9d added special handling image elements,
and for Cite references. This inadvertently broke handling of sorting
for tables with colspanned cells, which have node = undefined in the
cache.

In 2013, commit e203fdfc74f18 restored handling of node = undefined.

In 2015, commit 7a5f4e46d10ea made colspan expanded in the cache
just as rowspan was already. Thus making this code dead.

It's odd to see $(node) before this check, and even a call within
this conditional. This worked, as long way to return ''.

Change-Id: Ia823a3ec29371be39fab3d626ed8647ee469951b