clang build/mac: Stop forcing Py2

Upstream now requires Py3, so let's see if the bot update to 10.15
happened to fix crbug.com/1037896...

Bug: 1160953,1037896
Change-Id: Ib38ee655411c21552521b04e7852e16e5e2515b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595261
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838714}

tools/clang/scripts/build.py

@@ -571,8 +571,6 @@ def main():
         '-DLIBCXX_INCLUDE_TESTS=OFF',
         '-DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF',
     ])
-    # Prefer Python 2. TODO(crbug.com/1076834): Remove this.
-    base_cmake_args.append('-DPython3_EXECUTABLE=/nonexistent')
 
   if args.gcc_toolchain:
     # Force compiler-rt tests to use our gcc toolchain (including libstdc++.so)