RFR: Do not mention GC in Partial GC ops

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concpartial-drop-gc/webrev.01/

As Christine mentions separately, having a "GC" line in logs that does not decrease heap occupancy
might be perceived as the bug. Let us partially (pun intended) manage that by dropping "GC" from
those lines, so that concurrent phase would look like a part of larger cycle.

[13.341s][info][gc     ] GC(3) Pause Init Partial 2.229ms
[13.363s][info][gc     ] GC(3) Concurrent partial 21420M->21556M(102400M) 21.912ms
[13.364s][info][gc     ] GC(3) Pause Final Partial 0.914ms
[13.373s][info][gc     ] GC(3) Concurrent cleanup 21560M->1132M(102400M) 9.714ms

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey