Remove config that sets pre-push hook

[aasthayadav]

Current:
Repo owners who do not set "uber.diff.git.push.verify": true by default are not running pre-push hook on git push.
This is not the default in git and just like all other git hooks, pre-push hooks should run.

After this change:
"uber.diff.git.push.verify" config would not let you override running pre-push hooks.

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[uberSri]

default behavior should verify. Lgtm except for comment.