Detaches `oldBlock' from the list of NxsBlock objects growing from `blockList'.
If `blockList' itself points to `oldBlock', this function sets `blockList' to point to `oldBlock->next'. Note: the object pointed to by `oldBlock' is not deleted, it is simply detached from the linked list. No harm is done in Detaching a block pointer that has already been detached previously; if `oldBlock' is not found in the block list, Detach simply returns quietly. If `oldBlock' is found, its SetNexus object is called to set the NxsReader pointer to NULL, indicating that it is no longer owned by (i.e., attached to) a NxsReader object.