RFR (S): 8199326: Remove G1 gc time stamp logic

[Thomas Schatzl]

Hi,

  with the rebuild remsets concurrently changes the last user of the gc
time stamps (create live data) has gone away.

This change removes all of the corresponding code I could find.

Could you review it?

CR:
https://bugs.openjdk.java.net/browse/JDK-8199326
Webrev:
http://cr.openjdk.java.net/~tschatzl/8197932/webrev/
Testing:
hs-tier1

Thanks,
  Thomas