Revert bundler_ext commits.

b3d6bd6c: bundler_ext integration (rev 3)
0276a23e: Warn and fallback to bundler when bundler_ext variables are not set.

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In upstream, we don't use bundler_ext because we use rubygems.org for gems directly.
In downstream, we provide a Gemfile.lock on the shipping appliances.  The lock file ensures our shipping bundle does not get polluted with rubygems.org based gems such as ones used in automate ruby methods.  Therefore, bundler_ext provides no added benefit over what's provided in bundler for our purposes.