Remove unused method _get_res_pool_ref from VMware

This method has been moved to vm_util.py in commit
9408d79e7413aadcba52a61b7a33164090785e4f, so we can
remove it safely.

Change-Id: I6c192414c58c30ceec206937694ae92a3eea4d2d
Merge "Remove unused method _get_res_pool_ref from VMware"