Remove dead code from test_get_volume_stats()

The _emulate_vgs_execute() routine is never called because the function
that calls it (get_all_volume_groups) is overridden to return fake
output without executing vgs.

Change-Id: I3d3f536f7d85128547dc9e758e3930412ebed121
Merge "Remove dead code from test_get_volume_stats()"