Remove broken ShellAppSystem API and all consumers

Remove broken ShellAppSystem API and all consumers

In commit 9bd22dc0, I introduced an API to load an arbitrary
.desktop file, not necessarily from the menu path.  It turns
out this function was broken because it created ShellApp instances
that were *different* from ones that were cached normally.

As far as I can tell, we didn't initially use it.  Then later
Util.spawnDesktop was created which used this function.

Remove this broken function and all callers; if we're loading
.desktop files from *outside* the menu path, we can look at
readding.

This patch also kills off Util.spawnDesktop in favor of callers
talking to ShellAppSystem directly, now that the latter reports
errors.

https://bugzilla.gnome.org/show_bug.cgi?id=644402