[MLIR] Remove dead affine.applys while generating pointwise copies

This makes no impact on the test cases because affine-data-copy-generate
runs whole function canonicalization at its end; however, the latter
will be removed in a pending revision. It is thus useful to clean up
these affine.applys right here, and eventually, not even generate
these (when the right API to compose by construction is in place).

Differential Revision: https://reviews.llvm.org/D78055