Fix build warnings about unused variable 'buildService', and unnecessary 'try await'.

This now matches similar methods in the same file by simply returning a StringListResponse with an empty array.

rdar://122969732

Author: mhrawdon

Sources/SWBBuildService/Messages.swift

@@ -99,12 +99,10 @@ private struct MacCatalystUnavailableFrameworkNamesHandler: MessageHandler {
     }
 }
 
+// TODO: Delete once all clients are no longer calling the public APIs which invoke this message
 private struct AppleSystemFrameworkNamesHandler: MessageHandler {
     func handle(request: Request, message: AppleSystemFrameworkNamesRequest) async throws -> StringListResponse {
-        guard let buildService = request.service as? BuildService else {
-            throw StubError.error("service object is not of type BuildService")
-        }
-        return try await StringListResponse([])
+        return StringListResponse([])
     }
 }