[Catapult] Unblock Build Python Tests in Linux PY3 try job.
[Catapult] Unblock Build Python Tests in Linux PY3 try job.

Removing the PYTHONPATH to avoid the import error.

bug: chromium:1233829
Change-Id: I08ca1d9256cae208eb60bc3be8c26a3c009155b8
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3058510
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Commit-Queue: Wenbin Zhang <45ad8a1ff616a38a2202fd35571df81897c5f14f@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult