[mlir][linalg] Remove generic PadTensorOp vectorization pattern

The generic vectorization pattern handles only those cases, where
low and high padding is zero. This is already handled by a
canonicalization pattern.

Also add a new canonicalization test case to ensure that tensor cast ops
are properly inserted.

A more general vectorization pattern will be added in a subsequent commit.

Differential Revision: https://reviews.llvm.org/D103590