MM-44638: Remove upgrade.go (#20309)

We remove any remaining migrations from the upgrade.go file

```release-note
NONE
```

testlib/store.go

@@ -25,6 +25,7 @@ func GetMockStoreForSetupFunctions() *mocks.Store {
 	mockStore := mocks.Store{}
 	systemStore := mocks.SystemStore{}
 	systemStore.On("GetByName", "FirstAdminSetupComplete").Return(&model.System{Name: "FirstAdminSetupComplete", Value: "true"}, nil)
+	systemStore.On("GetByName", "RemainingSchemaMigrations").Return(&model.System{Name: "RemainingSchemaMigrations", Value: "true"}, nil)
 	systemStore.On("GetByName", "ContentExtractionConfigDefaultTrueMigrationComplete").Return(&model.System{Name: "ContentExtractionConfigDefaultTrueMigrationComplete", Value: "true"}, nil)
 	systemStore.On("GetByName", "UpgradedFromTE").Return(nil, model.NewAppError("FakeError", "app.system.get_by_name.app_error", nil, "", http.StatusInternalServerError))
 	systemStore.On("GetByName", "ContentExtractionConfigMigrationComplete").Return(&model.System{Name: "ContentExtractionConfigMigrationComplete", Value: "true"}, nil)