RootCanal: Remove option --default_commands_file

The only use case for the option was to add
beacon devices in cuttlefish for passing
LeScanning CTS tests. As short term solution,
pending a better one (test channel), one beacon
is attached to rootcanal by default.

Bug: 281083061
Test: m root-canal
Change-Id: I495e1cca69ae85fd9c8937a3e9ea0a9ca808789e