[MLIR][AMX] Remove llvm.ptr<i8> bitcasts from legalize for LLVM pass

This commit removes the no longer required `llvm.ptr<i8>` bitcasts from
AMX's legalize-for-llvm-export pass. Typed pointers have been
deprecated for a while now and it's planned to soon remove them from
the LLVM dialect.

Related PSA: https://discourse.llvm.org/t/psa-removal-of-typed-pointers-from-the-llvm-dialect/74502