Brick LVM: Remove self.lv_list

This member isn't used anywhere, and now that requests for
specific LVs are overwriting self.lv_list with a subset of
the LVs that are present, it doesn't contain coherent info
anyway.

Change-Id: I4d661869012937cc40177e53c8546d5cd7c98cd9
Merge "Brick LVM: Remove self.lv_list"