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With crrev.com/c/2637092, "package_paths" prop is removed so there is
no need to keep packager_vars.star.

Also set the package_prefix for trybot to "chromium_3pp".
Currently 3pp uploads package sources to CIPD, even with "force_build"
set to True (Behavior to-be-changed). With this package_prefix, sources
will be uploaded to CIPD path "experimental/chromium_3pp"

Bug: 1132368
Change-Id: I3b3e7cc36badda7687da967c2e4681bfcea6187f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639229Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#845421}