Revert "Updated to dynamically generate Overseer URL for JWTs"

This reverts commit 8c05cdab4860703d2870f42efe0acfc49732212f.

I prefer it to have get the overseer's URL from the config file rather
then indirectly through the token. Configuring that URL on every server
instance does not save any work since the server's own API auth token
must still be configured anyway.
Remove issuer field from JWTs.

Nothing reads this field now that the server behavior is reverted.
Authenticity of the tokens is provided via the PKI signature check
when tokens are verified. The issuers identify is clear in that case
and can be easily retrieved from the signing certificate.