Remove support for ro.audio.silent

Tradefed doesn't handle ro properties well, and test devices can be
left in unexpected states.

Remove and see what happens.

Bug: 375939958
Test: Manual
Flag: EXEMPT trivial
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:967c85fe01ab6acf8ab2ae54569ab90ef7737424)
Merged-In: Ibf16b8653a9fdf1e3f49b62b1247939b88620e43
Change-Id: Ibf16b8653a9fdf1e3f49b62b1247939b88620e43