This reverts commit c40a21f3.

Reason for revert: Seems to be failing locally on Windows.
See bug for more info.

Original change's description:
> Add export_targets.py to presubmit
> 
> Adds export_targets.py to run as part of presubmit in order to help
> prevent breaking Firefox with BUILD.gn changes.
> 
> Bug: chromium:1003151
> Change-Id: I5a7ab00891cd7c094c797e6150f642f803a726b6
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1802038
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>

TBR=ynovikov@chromium.org,bsheedy@chromium.org,jgilbert@mozilla.com,jmadill@chromium.org

Change-Id: Ic9a2eb58d847a40349e64b931c369f07c24b799c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1003151
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1841877Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>