This reverts commit 192da9d7.

Reason for revert: Causing failures on mac builders due to license.

Original change's description:
> Use newer OSX SDK for iOS
> 
> Based on http://crrev.com/c/2056465
> 
> Bug: chromium:1052887
> Change-Id: I2f76067883ff528ca45becd90dd3a6f71a507494
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2059669
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>

TBR=thakis@chromium.org,ynovikov@chromium.org,jmadill@chromium.org

Change-Id: I8698ed56fc81a76900da2aaf6cd19d08b57c5525
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1052887
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2061228Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>