Remove Kubeflow Training imports from go.mod

[andreyvelich]

As we discussed in our WG community meeting we can remove packages for kubeflow/common and kubeflow/tf-operator from Katib controller.
That will help us to decrease dependencies.
We are using this only in Validation webhook: https://github.com/kubeflow/katib/blob/master/pkg/webhook/v1beta1/experiment/validator/validator.go#L350-L371 which is not necessary.

/cc @gaocegege @johnugeorge
/area release
/priority p1

/assign @andreyvelich

[gaocegege]

SGTM!