Replace $timeline-image-border-radius with a CSS custom property (#10792)

* Use CSS custom property: --timeline-image-border-radius

Remove the same variable from _MImageBody.pcss

* Rename the property

The property is neither limited to a timeline nor image, but applied to M.*?Body components as well, such as MLocationBody and MVideoBody.

res/css/components/views/messages/_MBeaconBody.pcss

@@ -20,7 +20,7 @@ limitations under the License.
     max-width: 325px;
     width: 100%;
 
-    border-radius: $timeline-image-border-radius;
+    border-radius: var(--MBody-border-radius);
     overflow: hidden;
 }