RFR: 8246622: Remove CollectedHeap::print_gc_threads_on()

[Per Liden]

Instead of having all GCs implement 
CollectedHeap::print_gc_threads_on(), we can just let the single caller 
(Threads::print_on) provide a closure and use 
CollectedHeap::gc_threads_do(). That will better match what 
Threads::print_on_error() is already doing, and remove repetitive code 
in the GCs.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246622
Webrev: http://cr.openjdk.java.net/~pliden/8246622/webrev.0

/Per