remove unnecessary default prop from test (#7984)

See https://github.com/WordPress/gutenberg/pull/7773/files#r202718599 for comment raising this issue.  There is no necessity for this prop to be set because we aren’t testing anything explicitly requiring that prop.  Originally it was intended to mock the `instanceId` prop provided by the `withInstanceId` HOC but was implemented with an incorrect name.  As mentioned, its not needed at all so just removing.

editor/components/inserter/test/menu.js

@@ -93,7 +93,6 @@ const items = [
 
 const DEFAULT_PROPS = {
 	position: 'top center',
-	instanceID: 1,
 	items: items,
 	debouncedSpeak: noop,
 	fetchSharedBlocks: noop,