Cleanup CI decomposition phase 4 feature flag and env vars

What does this MR do and why?

This removes the force_no_sharing_primary_model feature flag and the GITLAB_LOAD_BALANCING_REUSE_PRIMARY_ environment variable (used as GITLAB_LOAD_BALANCING_REUSE_PRIMARY_ci). These were used in Phase 4 which is rolled out in gitlab-com/gl-infra/production#6440 (closed) so we can now remove these.

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Closes #357635 (closed)