Remove use of equality_constraints (#1472)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1472

With improvements to the dynamic_shapes API, dynamic dimensions which
are equal (they are marked with the same named `Dim`) will have the same symint
assigned to them. So, we no longer need `equality_constraints` to tell us which
inputs dimensions are equal.

Reviewed By: zhxchen17

Differential Revision: D52314640

fbshipit-source-id: a8ef0c8074229da2c6e8550077e8087b737fec92