Remove config parsing logic that is now implemented in the CLI

[henrymercer]

In CodeQL CLI v2.11.6, we finished implementing key parts of parsing the code scanning configuration within the CLI. This enabled this functionality to be used by folks running code scanning using a third-party CI system.

However due to the need to support earlier versions of the CLI that do not support config parsing, we have needed to duplicate config parsing code between the Action and the CLI.

Now that CodeQL CLI v2.11.5 and earlier have been deprecated, we can remove this legacy config parsing code from the Action.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[angelapwen]

Whew, I think I 👀 d all the changes — looks so much cleaner now! I assume the status report changes will be a follow-up?

[henrymercer]

There are two aspects to the status report changes:

1. Renaming builtin to all — I think this is fairly low priority and can happen as follow up
2. Removing the custom fields on the server-side — I think it might make sense to keep these around a little longer so we still receive the full telemetry from earlier Actions, so we'll do this as follow up too.