Remove bootstrapping workaround for ListRoleAssignments

Previously ListRoleAssignments needed to be able to return the default
assignments when the metadata client was not initialized. This was added
in order to be able to bootstrap the metadata client (which caused a
call to ListRoleAssignments by itself). With the recent switch to a
separate user-provider and permissions service for the metadata storage
this no longer needed.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f