Removing redundant setTimestamp call

As per the documentation, client supplied values are ignored and the timestamp is set when the log is persisted.
This is purely a cosmetic cleanup and doesn't change behaviour.

Bug: 64914512
Test: Confirmed in MetricsReader impl that timestamp is overridden, and tested logcat output for regressions.
Change-Id: Ia7db6410f82f08fe501607def0a87925a6aa9329