Statsd: Remove storage of bucketNum

Past buckets had a field mBucketNum, storing the
"bucket number" they represented. Currently, the concept
of bucket number is used by Anomaly Detection (although this
may not be necessary), but these stored values are not used at all,
so removing them will save RAM.

Test: statsd tests still pass
Bug: 74607818
Change-Id: Iacc343bc39c5035f6e2f236c03de1d91606eff4c