removing line about custom themeing cuz it doesnt really work (#2834)

Author: LFDanLu

packages/dev/docs/pages/react-spectrum/theming.mdx

@@ -110,7 +110,3 @@ import {theme} from '@react-spectrum/theme-default';
   <YourApp />
 </Provider>
 ```
-
-You can also build your own custom themes and apply them the same way. See the
-[source code](https://github.com/adobe/react-spectrum/tree/main/packages/%40adobe/spectrum-css-temp/vars)
-for the default theme for a list of variables that should be defined.