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ARROW-7828: [Release] Remove SSH keys for internal use

These SSH keys are only used to login to local Docker container for
releasing binary artifacts. They aren't used over network. So putting
them to this public repository is safe.

But there are people who report "they may be danger" to us. We should
remove them from this repository and generate them locally instead of
describing why they aren't danger to reporters.

Closes #6399 from kou/release-remove-ssh-key and squashes the following commits:

65d0ffcd3 <Sutou Kouhei>  Remove SSH keys for internal use

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>