Stop explicitly setting use_prebuilt_instrumented_libraries on msan bots.
Stop explicitly setting use_prebuilt_instrumented_libraries on msan bots.

After https://chromium-review.googlesource.com/c/chromium/src/+/1797468
(rolled in https://webrtc-review.googlesource.com/c/src/+/152601) it's set by
default in is_msan=true builds.

Bug: webrtc:10967
Change-Id: I380ef3bf1cfdc2aba983c8506e27e3a6b2868e6d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152720
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Reviewed-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29171}

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