aconfig: remove support for --format=text

This CL is part of a larger effort to simplify maintenance by removing
format aliases: with the new --format option, clients of `aconfig dump`
can specify the format they need themselves.

This CL removes the last of the format aliases.

Bug: 317044914
Test: m
Change-Id: I09b706b9931a85a86c17be359902245b9b53d584