Remove data dropping as an invalid bucket case

ValueMetric's #dropData function flushes and clears the current bucket
only if the drop time coincides with the end of the bucket. Otherwise,
if the current bucket is a partial bucket, we keep this bucket for
further aggregation so it should not be marked invalid.

Test: bit statsd_test:*
Bug: 145918755
Change-Id: I73bccf8fe9e223bd7927fffe2f1665744ae1b452