remove dead code from vm_map_entry_unlink

[dougm]

Summary

The UNLINK_MERGE_PREV unlink_merge_type is no longer used, so drop it and the code associated with it.

[alc]

When you commit the log entry should: (1) reference the prior commit when this code became dead code and (2) say, "Tested by: pho (as part of a larger change)"