Remove old unused code from PDFView.close()

This patch removes a piece of dead code from `PDFView.close()`.

The code removed here originated in PR #285, but hasn't actually been used for anything in a long time. Furthermore it appears that this code was already obsolete when the current viewer UI was first introduced in PR&nbsp;#1537.

(As an aside: Looking at PR #285 I'm actually wondering if it ever worked as intended, since the property is called `_interval` in one place and `_loadingInterval` in another :-)