Clean up oidc_issuer_url feature flag

What does this MR do and why?

The MR removes the feature flag for the following fix:

The GitLab OIDC and ID Token issuer attributes did not match when the self-hosted domain is not the root of the domain. This change uses the same issuer attribute for both.

Related to #429855 (closed)

Added feature flagskipped because we are removing the feature flag.

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