fix(contextualhelp): use spectrum-tokens var for font-size

Following the fix in `@adobe/[email protected]`, this
removes a unnecessary custom property definition created within
the ContextualHelp component, and instead, uses the custom
property as generated directly from Spectrum Tokens.

Ultimately, this resolves an issue that was causing the font-size
value for the Contextual Help's body to be incorrect.

Author: pfulton

components/contextualhelp/index.css

@@ -18,7 +18,6 @@ governing permissions and limitations under the License.
 
   /* Typography Variables */
   --spectrum-contextual-help-heading-size: var(--spectrum-contextual-help-title-size);
-  --spectrum-contextual-help-body-size: var(--spectrum-contextual-help-body-size);
   --spectrum-contextual-help-heading-color: var(--spectrum-heading-color);
   --spectrum-contextual-help-body-color: var(--spectrum-body-color);