1.) Remove unused zlib contrib/ directories
- There are missing licenses in third_party/zlib/contrib/, so remove the
unused directories from the AOSP checkin.

2.) `git add` all third_party/ changes
- All of the unused third_party/ changes have been manually merged, so
we can now automatically add any changes to third_party/.

Bug: b/191882454
Bug: b/187342779
Change-Id: If209ff3bb34ad53c08f431a06a2f84d153181e45
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2992578Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>