Remove code related to stolen trace sessions

As of aosp/2323319, tracing_service_proxy will always report
session_stolen=false. This change updates TracingServiceProxy.java to
reflect this.

Test: Check that System Tracing + bug report interactions are still
      correct.
Bug: 352779024
Flag: EXEMPT no-op cleanup
Change-Id: I9b199c3040db47cdf1c6ef7cd03567e785a63b17
Merged-In: I9b199c3040db47cdf1c6ef7cd03567e785a63b17