RFR: 8078897: Clean out unused code in G1MMUTracker

[Stefan Johansson]

Hi,

Please review this small cleanup:
https://bugs.openjdk.java.net/browse/JDK-8078897

Webrev:
http://cr.openjdk.java.net/~sjohanss/8078897/hotspot.00/

Summary:
Removed the call to longest_pause since the retrieved value was never used.

Thanks,
Stefan