Remove old onBackground attribute from sidenav storybook story (#671)

* fix(sidenav): remove onBackground attribute

When merging the latest develop into this release branch the sidenav
component ended up with both the old onBackground and new isOnBackground
versions of the attribute in its storybook story. This removes the old
version so it is correct.

* chore: add changeset

.changeset/eleven-spoons-shop.md

@@ -0,0 +1,5 @@
+---
+'@ithaka/pharos': patch
+---
+
+Remove deprecated onBackground attribute from sidenav storybook story

packages/pharos/src/components/sidenav/PharosSidenav.react.stories.jsx

@@ -62,7 +62,6 @@ export const Base = {
             variant="subtle"
             isOnBackground
             a11yLabel="search"
-            onBackground
           ></PharosButton>
         </PharosInputGroup>
         <PharosSidenavSection showDivider>