Remove "Already signed in with SAML"

What does this MR do and why?

This message is displayed when a user authenticates with SAML and their SAML identity is already linked to their GitLab account. This is the expected state for every single SAML sign-in after the first one. It doesn't make sense to show a message for this since it is the normal behavior. Instead, we should simply redirect them to whatever page they were trying to get to and show nothing.

To observe this behavior on GitLab.com, follow this link: https://gitlab.com/groups/gitlab-com/-/saml/sso

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