RFR: 8210714: ZGC: ZWeakRootsIterator should no longer call reset/finish_dead_counter()

[Per Liden]

When the ZConcurrentStringTable option was removed (JDK-8210061) and 
that behavior was made the default and always on, we left calls to 
StringTable::reset/finish_dead_counter() that are no longer needed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210714
Webrev: http://cr.openjdk.java.net/~pliden/8210714/webrev.0

/Per