cleanup(sidekick/config): remove support for conformance-root (#4491)

googleapis/google-cloud-rust#4921 removes the
last root_name that has the -root suffix, so no longer need to support.

Author: jameslynnwu

internal/sidekick/config/source_config.go

@@ -61,7 +61,7 @@ func (c SourceConfig) Root(name string) string {
 		return c.Sources.Showcase
 	case "protobuf-src":
 		return c.Sources.ProtobufSrc
-	case "conformance", "conformance-root":
+	case "conformance":
 		return c.Sources.Conformance
 	default:
 		// Unknown root name