Remove unused variable

o ceilometer/compute/virt/hyperv/utilsv2.py:    _AGGREG_METRIC
  = 'Msvm_AggregationMetricDefinition'

  This variable was added in the following commit.

    commit 81e336a376d8a6e61a9a7bb5c686ab22ad38ee0d
    Author: Claudiu Belu <cbelu@cloudbasesolutions.com>
    Date:   Thu Aug 22 09:59:51 2013 -0700

        Adds Hyper-V compute inspector

  But there is no evidence that was used.

Change-Id: I417d6a249cb196d4e9e988d6d93d91623671f7af
Merge "Remove unused variable"