gui: Fix unsuccesful attempt at loading default viewer iconset
The default viewer icons file seems to have
been removed in the rewrite of buildzip.pl
(see commit 66b6fdb).
So, this only worked accidentally, once some
theme put a viewers.bmp file in the icons folder,
potentially leading to confusing behavior.
Deactivating viewer icons (instead of reverting
to some default file) when they are set to "-"
is probably expected behavior at this point.
Change-Id: I5010764676c67592bf20abfb3d0780edb1d555d4