Remove unf gem from Gemfile

What does this MR do and why?

The unf gem was initially introduced in e1f6fa6f, along with the introduction of fog gem for uploading to AWS storage purposes.

This gem is no longer used by the fog gem https://github.com/fog/fog/commit/8426fc9abf10c0ce2b039d26830e000361dac604.

Note that the gem is still used by domain_name, so it will remain in Gemfile.lock.

Resolves #424715 (closed)

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