Remove timestamps from docs/api-reference/*/*.html

If there are 2 or more PR(s) in the queue, they will end up with
conflicts (and rechecks). So let us remove the timestamp entirely
when we generate the files.

Fixes #46814

docs/api-reference/extensions/v1beta1/definitions.html

@@ -8224,7 +8224,7 @@ Both these may change in the future. Incoming requests are matched against the h
 </div>
 <div id="footer">
 <div id="footer-text">
-Last updated 2017-06-10 04:37:41 UTC
+
 </div>
 </div>
 </body>

docs/api-reference/extensions/v1beta1/operations.html

@@ -13942,7 +13942,7 @@ span.icon > [class^="icon-"], span.icon > [class*=" icon-"] { cursor: default; }
 </div>
 <div id="footer">
 <div id="footer-text">
-Last updated 2017-05-17 22:25:20 UTC
+
 </div>
 </div>
 </body>