applet: don't autostart the applet in GNOME any more

Applet autostart was disabled by d6ffe34d
but re-enabled by e192e83c after working
out some issues with the GNOME shell watcher code.  The only function
of the applet's agent after that point was handling WWAN PIN auto-unlock,
which should be done differently per 4473e6a4.
Thus autostart is no longer needed because GNOME Shell already handles
all the necessary agent functionality.