settings: drop unused v1.1 mirror setter and constant

EnableV1CustomRefMirror() and checkpointsVersionV1CustomRef had no production callers — the setter was used only by dispatch tests and the constant only by that setter plus a one-line refactor of isV1CustomRefValue. The local v1.1 read path consumes the existing MirrorsToV1CustomRef getter, so none of it is needed. Revert settings.go to its main state.

Tests set the checkpoints_version strategy option directly via a shared mirrorEnabledSettings() helper, keeping the "1.1" on-disk encoding in one place.