physics: Delete `StopAt::EmptySpace`. #179

Job	Run time
miri	2m 9s
no_std	2m 12s
build (ubuntu, stable, locked, true)	1h 11m 4s
build (windows, stable, locked, false)	22m 34s
build (macos, stable, locked, false)	20m 22s
build (ubuntu, nightly, locked, false)	15m 23s
build (ubuntu, beta, locked, false)	17m 46s
build (ubuntu, stable, latest, false)	14m 54s
build (ubuntu, stable, minimal, false)	29m 22s
deploy	8s
fuzz	19m 16s
	3h 35m 10s

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