Remove unused code from volume manager (reset_stats)

self._last_volume_stats appears to be unused, now that we don't
use _volume_stats_changed().  This means reset_stats() also does
nothing.

This triggered a pylint 'attribute-defined-outside-init' warning.

Change-Id: I4c4f7497e72f726f3430f90df61188bf2f1073c1
Merge "Remove unused code from volume manager (reset_stats)"