RFR: Cleanup reset_{next|complete}_mark_bitmap

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/cleanups-7/webrev.01/

Trivial cleanup: these methods are always called with workers(), we might as well inline.

Testing: hotspot_fast_gc_shenandoah

Thanks,
-Aleksey