msan: Stop explicitly setting use_prebuilt_instrumented_libraries.

It's now set by default if is_msan is set.

Bug: v8:9715
Change-Id: I84e05ff9a495b666292891a12a3ebe485e4a768a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1810558
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63887}

infra/mb/mb_config.pyl

@@ -670,14 +670,12 @@
 
     'msan': {
       'mixins': ['v8_enable_test_features'],
-      'gn_args': ('is_msan=true msan_track_origins=2 '
-                  'use_prebuilt_instrumented_libraries=true'),
+      'gn_args': 'is_msan=true msan_track_origins=2',
     },
 
     'msan_no_origins': {
       'mixins': ['v8_enable_test_features'],
-      'gn_args': ('is_msan=true msan_track_origins=0 '
-                  'use_prebuilt_instrumented_libraries=true'),
+      'gn_args': 'is_msan=true msan_track_origins=0',
     },
 
     'msvc': {