[mlir] Toy tutorial: avoid erasing and then re-creating loop terminators

The lower-to-affine-loops pass in chapters 5-7 of the Toy tutorial has
been creating affine loops, erasing their terminator and creating it
anew using a PatternRewriter instance to work around the fact that
implicit terminators were created without notifying the rewriter. Now
that has been fixed in 3ccf4a5b, remove the code erasing and
re-creating the terminators and rely on the default ones.