Change gcstress runtest option

No longer passes down SS as the collector. Motivation is to allow
running gcstress with concurrent copying.

New usage is for SS:
mm test-art-host ART_TEST_GC_STRESS=true ART_DEFAULT_GC_TYPE=SS

Bug: 30807718

Test: mm test-art-host ART_TEST_GC_STRESS=true ART_DEFAULT_GC_TYPE=SS

Change-Id: I4416c1ca9ead027536acc9d24b1ec80d5a68c6a1
1 file changed