master.lmde6

mate-xapp-status-applet.py: Don't use theme.has_icon() - not all

usable icons are in themes.

Icon files in /usr/share/xpm will not show up in theme.has_icon(),
but will still be found when creating a GtkThemedIcon (or otherwise
using the icon name).

There's not much point in this check anyhow - if the icon really
doesn't exist, there will still be a fallback icon shown.

Fixes linuxmint/mint-x-icons#209.