refactor(dropzone): update values for pointer-events custom prop

This removes the `--mod-*` prefixed custom properties for pointer-events used
within Drop Zone-nested Illustrated Messages. These custom properties were
preventing interactions within the Illustrated Message component.

Instead, over in Illustrated Message, we'll keep the `--mod-*` custom props
available in the stack defined for `pointer-events`, but we'll set a fallback
value of `auto`, which will preserve the ability for consumers to use the mod
but also get the default style if it is not set.

Author: pfulton

components/dropzone/index.css

@@ -56,7 +56,6 @@ governing permissions and limitations under the License.
   --spectrum-drop-zone-content-color: var(--spectrum-white);
 
   /* Settings for a nested illustrated message */
-  --mod-illustrated-message-pointer-events: none;
   --mod-illustrated-message-content-maximum-width: var(--mod-drop-zone-content-maximum-width, var(--spectrum-drop-zone-content-maximum-width));
   --mod-illustrated-message-illustration-color: var(--mod-drop-zone-illustration-color, var(--spectrum-drop-zone-illustration-color));
   --mod-illustrated-message-title-to-heading: var(--mod-drop-zone-illustration-to-heading, var(--spectrum-drop-zone-illustration-to-heading));
@@ -69,7 +68,6 @@ governing permissions and limitations under the License.
   --mod-illustrated-message-title-color: var(--mod-drop-zone-heading-color, var(--spectrum-drop-zone-heading-color));
   --mod-illustrated-message-description-position: relative;
   --mod-illustrated-message-description-z-index: 10;
-  --mod-illustrated-message-description-pointer-events: initial;
   --mod-illustrated-message-heading-to-description: 0;
   --mod-illustrated-message-description-font-family: var(--mod-drop-zone-body-font-family, var(--spectrum-drop-zone-body-font-family));
   --mod-illustrated-message-description-font-weight: var(--mod-drop-zone-body-font-weight, var(--spectrum-drop-zone-body-font-weight));