Reconsider "application data" field on LWE ciphertext

Now that we have (nearly) completed the packing system, the `application_data` attribute on an LWE ciphertext type is currently only used for the debug handler framework, so that the debug handler can understand how to unpack to print cleartext values.

With the changes in https://github.com/google/heir/pull/2199 to use multi-packed ciphertexts by default, the best we will be able to do with this attribute is to print the packed cleartext values (because the remainder of the logic of how to unpack is not present on the LWE ciphertext type).

We should remove the application data field, and instead adapt the debug handler framework so that it is aware of the new layout system, and insert `secret.reveal/tensor_ext.unpack` ops appropriately that can be lowered automatically.