Remove `omniauth-kerberos` gem

What does this MR do and why?

This MR removes the deprecated omniauth-kerberos gem.

Related to #2908 (closed).

We remove omniauth-kerberos to remove support for password-based Kerberos sign-ins, see https://docs.gitlab.com/ee/integration/kerberos.html#upgrading-from-password-based-to-ticket-based-kerberos-sign-ins.

"Kerberos SPNEGO" authentication retains to be supported.

https://gitlab.com/gitlab-org/gitlab/-/blob/4ed4e07dd9a5eb25f345b99b589b091254c9031c/ee/app/controllers/ee/omniauth_callbacks_controller.rb#L13

Note that "Kerberos SPNEGO" and omniauth-kerberos share common code in the codebase so it is quite hard to identify which part of code is dead code due to removing omniauth-kerberos. I deleted the minimum to prevent regression.

Partially reverts 8a799319.

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