[css-forms-1][editorial] Use empty alt if figure caption suffices

Author: svgeesus

css-forms-1/Overview.bs

@@ -1057,24 +1057,24 @@ especially on mobile devices where they're a bit "weirder".
 ### Time Pickers ### {#time-examples}
 
 <figure>
-  <img src="images/time01.png">
+  <img src="images/time01.png" alt="">
   <figcaption>iOS time picker</figcaption>
 </figure>
 
 <figure>
-  <img src="images/time02.png">
+  <img src="images/time02.png" alt="">
   <figcaption>Android time picker 1</figcaption>
 </figure>
 
 <figure>
-  <img src="images/time03.png">
+  <img src="images/time03.png" alt="">
   <figcaption>Android time picker 2</figcaption>
 </figure>
 
 ### Date Pickers ### {#date-examples}
 
 <figure>
-  <img src="images/date01.png">
+  <img src="images/date01.png" alt="">
   <figcaption>Android date picker</figcaption>
 </figure>