Fixes most nullspace items, rewrites floor tiles not to del() so much

Not sure of exact numbers on dev, but on master there were 11670 nullspace `/obj` (not counting `/obj/screen`) on a round-start map, of which approximately 11400 were nullspace floortiles that only ever got used as state containment boxes for light floors' on/off and repair state - not even reused when prying up the tiles again.

Current nullspace `/obj/item` at roundstart as of this PR:
* Nine radio headsets `/obj/item/device/radio/headset`
* One pair of meson goggles `/obj/item/clothing/glasses/meson`
* One station intercom `/obj/item/device/radio/intercom` (set to response team freqs?)
Yeah, but that's probably been there *ages*.