Several Shenandoah source files contain duplicate semicolons (`;;`) at the
end of statements. While harmless, they are clearly typos.

Files:
- shenandoahAdaptiveHeuristics.cpp (1 instance)
- shenandoahGenerationalHeuristics.cpp (1 instance)
- shenandoahFreeSet.cpp (4 instances)
- shenandoahGenerationalControlThread.cpp (1 instance)