G1ConcurrentMark.hpp unnecessarily disables MSVC++ warning 4522.

JDK-8243326 eliminated the non-default copy constructor and copy assignment operators for G1TaskQueueEntry. There were two assignment operators, for normal and volatile entries, which would have been triggering MSVC++ warning C4522 "multiple assignment operators specified". However, that warning is disabled around the definition of that class. When the assignment operators were removed, the warning suppression should have been removed too, but was overlooked.