Migrate defaultLanguage(for:) to BSP

Sources/BuildSystemIntegration/BuildServerBuildSystem.swift

@@ -338,10 +338,6 @@ extension BuildServerBuildSystem: BuiltInBuildSystem {
     )
   }
 
-  package func defaultLanguage(for document: DocumentURI) async -> Language? {
-    return nil
-  }
-
   package func toolchain(for uri: DocumentURI, _ language: Language) async -> Toolchain? {
     return nil
   }