Resolve #3628096 "Default Admin attaches a library it doesn't define"

FormHooks::formMediaLibraryAddFormAlter() attached default_admin/media_library.theme, which is not defined in default_admin.libraries.yml (a leftover from Claro's claro/media_library.theme). Media library styling is already provided via libraries-extend of media_library/ui, view and widget with default_admin/media_library, so the attachment is removed.