aconfig: remove the 'debug' dump format

The --format=debug dump output format is largely superseded by the
textproto format now that aconfig doesn't use hand-written wrappers
around the auto-generated proto struct anymore. Remove the format to
reduce the risk of code rot.

Bug: 283910447
Test: atest aconfig.test
Change-Id: I6700fc4eafd3fa3a63952109c0105d34c7ffd98b