Thanks, TwistedTwigLeg, your solution also worked for me. So confirmed to work 2/01/2019.
Couldn't work this one out either haha. I originally tried to $Mob.queue_free() under the function game_over but it only
seemed to remove one. Copying and pasting lead the game to crash as it couldn't find a node named "Mob".
Confused, I searched how to print all the nodes by using print_tree() which lead me to discover that they were named @Mob@2, @Mob@3 etc. At that point, I was at a loss. Even if I had a bunch of $Mob.queue_free's, if the node I was looking to free was not there, it would crash the game.
Thanks Twisted to teaching me about for statements :).