Unused upcalls

[javadamiri]

The following three upcalls are not used anywhere:

mmtk-openjdk/openjdk/mmtk.h

Lines 92 to 94 in 75296ac

	void (*compute_static_roots) (void* trace, void* tls);
	void (*compute_global_roots) (void* trace, void* tls);
	void (*compute_thread_roots) (void* trace, void* tls);

They should be removed or a comment should be added to explain their future use.