Properly detach rocket sneakers when the player dies

This includes both death pits and switching to spectator.

While thinking about potential causes for the specbombs that still rarely occur, I thought to check on how Rocket Sneakers are handled when the player turns into a spectator. I found the block of code in P_KillMobj that handles cleaning trailing and orbiting items. I found that it does not handle Rocket Sneakers. This leaves the hnext list unaddressed when P_KillMobj is called and a rocket sneaker is held, which is similar to previous syncbomb issues involving rocket sneakers and eggboxes or shrink.

I'm not entirely sure if this will fix specbombs or not, but it also does no harm to clean them up properly on death. It might improve game stability.