RFR: 8231668: Remove ForceDynamicNumberOfGCThreads

[Aditya Mandaleeka]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8231668

Webrev:
  https://cr.openjdk.java.net/~adityam/8231668/

This removes all the ForceDynamicNumberOfGCThreads-related code and the test cases using it.

Note: this is my first patch since getting Author status, so please feel free to let me know if there's
anything wrong with how I created the webrev. Other friendly folks have been doing that for me
until now :).

Thanks,
Aditya