Stop creating update_engine_payload_key

Now the update_engine is able to read public keysfrom otacerts directly.
So the update_engine_payload_key is no longer needed.

Also remove the key replace in sign_target_files_apks.py. So we should
not use the new script to sign the old target files.

Bug: 116660991
Test: build the system image, unit tests pass
Change-Id: I9dae1f8b397f2b5efafed66a8faac1cb9087c741