[mlir-opt][nfc] Remove dead function decls

This removes 3 dead function decls from mlir-opt:
- registerTestLowerToNVVM - recently removed in #75775 when NVVM was
  productized.
- registerTestPreparationPassWithAllowedMemrefResults - removed in
  D90778 (f7bc5682).
- registerTestGenericIRVisitorsInterruptPass - added in D116230
  (8067ced1) but never existed. Pass is registered by
  registerTestGenericIRVisitorsPass.