Remove unnecessary detail where there's potential controversy

We could default to showing the first frame, or the last one shown, or
provide control over this (as discussed below). Either way, that's not
essential to the design, so let's distract ourselves for now.

Author: frivoal

image-animation-property/README.md

@@ -166,7 +166,6 @@ all values have the same effect (i.e. none).
     synchronized with other instances of the same image on the page
 * **paused**:
     the image animation does not run.
-    Instead, the first frame of the image is displayed.
 * **running**:
     the image animation plays,
     without synchronization with any other instance of the same image on the page.