Stop forcing /usr/bin to the front of the path

Summary:
This messes with python version selection. At least based on
the comments, it is no longer needed, as /opt/local/bin appears to be
empty on my devserver and on a sandcastle host.

D32337004 revealed the problem.  T105920600 has some more context.  I couldn't repro locally, so D32451858 is a control (which I expect to fail tests) to confirm that this fixes the test failures.

#utd-hermes

Reviewed By: neildhar

Differential Revision: D32451851

fbshipit-source-id: 4c9c04571154d9ce4e5a3059d98e043865b76f78

ReactCommon/hermes/inspector/tools/sandcastle/setup.sh

@@ -15,10 +15,6 @@ set -o pipefail
 THIS_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 ROOT_DIR=$(cd "$THIS_DIR" && hg root)
 
-# Buck by default uses clang-3.6 from /opt/local/bin.
-# Override it to use system clang.
-export PATH="/usr/bin:$PATH"
-
 # Enter xplat
 cd "$ROOT_DIR"/xplat || exit 1