Remove the undefined in the api (microsoft#258295)

Remove the undefined

The underlying implementation only ever returns an array.. so this makes sense to only return an array.

Author: TylerLeonhardt

src/vscode-dts/vscode.proposed.secretStorageKeys.d.ts

@@ -11,6 +11,6 @@ declare module 'vscode' {
 		/**
 		 * Retrieve the keys of all the secrets stored by this extension.
 		 */
-		keys(): Thenable<string[] | undefined>;
+		keys(): Thenable<string[]>;
 	}
 }