Removes ability to craft or find pipe SMGs

You can make a single-shot firearm from a pipe, wood, and nails, sure.  A submachine gun, though?  That falls smack into the "machine tools" issue we raised in the design doc.

I'm not sure how many folks use pipe SMGs, but at this point I'd like to stop their creation.  We've already eliminated the 9mm version from turrets.  This eliminates them from being crafted or spawning.