Remove legacy build config

[achilleas-k]

bootc-image-builder/bib/internal/buildconfig/config.go

Lines 17 to 23 in 7dd9bea

	// legacyBuildConfig is the json based configuration that was used in
	// bootc-image-builder before PR#395. It was essentially a blueprint
	// with just the extra layer of "blueprint". Supporting it still makes
	// the transition of existing users/docs easier.
	type legacyBuildConfig struct {
	Blueprint *json.RawMessage `json:"blueprint"`
	}

This was rather sloppily added in the very initial versions (even before it was called bootc-image-builder) and we kept it around for some backwards compatibility with our early documentation. We should retire it in favour of consistency and cleaner configuration handling.