infra: Remove osx_sdk cache. This is no longer necessary now that we use mac_toolchain.py. Bug: angleproject:5114 Change-Id: I0afac641d55e9be2bd04103301aec3080d3332c8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2823859 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>