Revert "util: Be more gracefull on the app_id"

This breaks telegram desktop with app-id org.telegram.desktop and a desktop file of org.telegram.desktop.desktop.

We only made that change to not change the behaviour of the end-session-dialog. All inhibitors there do have a proper app-id though so let's just live without not appending '.desktop' there either.

This reverts commit 2a376330.

Reported-by: Newbyte newbie13xd@gmail.com

Closes: #1010 (closed)

/cc @Newbyte