RFR: 8231996: ZGC: Replace ZStatisticsForceTrace with check if JFR event is enabled

[Per Liden]

Remove and replace the diagnostic flag ZStatisticsForceTrace with a 
check if JFR event is enabled. This flag was introduced as a safety 
measure back when sending JFR events was problematic in some contexts. 
This is no longer the case, so we can just let the 
default.jfc/profile.jfc control when those events should be sent.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231996
Webrev: http://cr.openjdk.java.net/~pliden/8231996/webrev.0

/Per