Remove package:platform from issue template (#149995)

Package is being transferred to the `dart-lang` github org: flutter/flutter#141808

Author: mit-mit

.github/ISSUE_TEMPLATE/9_first_party_packages.yml

@@ -56,7 +56,6 @@ body:
         - palette_generator
         - path_provider
         - pigeon
-        - platform
         - plugin_platform_interface
         - pointer_interceptor
         - process