sensors: Drop tests for Sensor.start()/stop() return types.

These tests were essentially verifying IDL compliance for some methods,
which is not very relevant in this context.

Change-Id: Ib103418bf03b8843eda90ae291dceb224cf9e4a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398918


Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805545}