[JVMCI] remove duplicates in vmStructs_jvmci.cpp

The following exports are duplicated in vmStructs_jvmci.cpp:

- declare_constant(CollectedHeap::Serial) \
- declare_constant(CollectedHeap::Parallel) \
- declare_constant(CollectedHeap::G1) \
- declare_constant(CollectedHeap::Epsilon) \
- declare_constant(CollectedHeap::Z) \
- declare_constant(CollectedHeap::Shenandoah)