Avoid installing deprecated Python 2 from Homebrew (swiftlang#160)

`brew install python@2` now fails for me locally due to Homebrew/homebrew-core#49796, not sure how it still runs fine on CI, probably due to caches not being cleaned up as frequently.

I hope relying on system-installed Python should work fine.

Author: MaxDesiatov

Diff for: utils/webassembly/ci-mac.sh

@@ -2,10 +2,8 @@
 
 set -ex
 
-brew install cmake ninja llvm python@2
-
-# Install six for python3 migration
-pip2 install six
+brew uninstall python@2 || true
+brew install cmake ninja llvm
 
 SOURCE_PATH="$( cd "$(dirname $0)/../../.." && pwd  )" 
 SWIFT_PATH=$SOURCE_PATH/swift