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 <mbonadei@webrtc.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#29171}
diff --git a/tools_webrtc/mb/mb_config.pyl b/tools_webrtc/mb/mb_config.pyl
index de92c51..c90a406 100644
--- a/tools_webrtc/mb/mb_config.pyl
+++ b/tools_webrtc/mb/mb_config.pyl
@@ -536,8 +536,7 @@
     },
 
     'msan': {
-      'gn_args': ('is_msan=true msan_track_origins=2 '
-                  'use_prebuilt_instrumented_libraries=true'),
+      'gn_args': 'is_msan=true msan_track_origins=2',
     },
 
     'no_clang': {