`SparseTensorVariable` should not subclass `TensorVariable`

[jessegrabowski]

Description

This came up in the context of #1674. We should consider reverting aesara-devs/aesara#142. Sparse matrices and dense tensor are conceptually different enough that it doesn't make sense for one to be a direct subclass of the other.

I think there is room for a new abstract superclass that could define a common API they both need to adhere to, but that would be beyond the scope of what this issue is asking for.