TEP-0114: Remove the v1alpha1.Run

[Yongxuanzhang]

We need to remove v1alpha1.Run as planned in TEP-0114: Custom Tasks Beta
This issue is part of #5863.
#5870 should also be closed once this issue is done.

[Yongxuanzhang]

/assign

[lbernick]

@Yongxuanzhang sorry to snag this from you, but I know you're out for a few days and want to get this into the v0.47 milestone