ModGC

Remove old is_sweep_pthread_p check in darray.h #2709

Job	Run time
check (default, macos-latest)	4m 55s
check (mmtk, release, macos-latest)	5m 52s
check (default, ubuntu-latest)	4m 27s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
	6h 20m 14s

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