Remove logs related to UsageStats string cache.

Since UsageStatsProto is not being used anymore other than for
backup/restore, there is no string cache (string pool) maintained in
memory. Because of this, there are a bunch of warnings being logged
every time usage stats data is backed up. Remove these logs since they
are simply noise now.

Bug: 143184490
Test: atest UsageStatsDatabase (no warnings seen during backup test)
Change-Id: Ibb0a053e51124087f44231f5fddf3d41f7e345d2