Remove htmlunit Buck target

Because we now include the htmlunit driver as a third
party dependency.

Author: shs96c

.buckconfig

@@ -22,7 +22,6 @@
 [alias]
   chrome   = //java/client/src/org/openqa/selenium/chrome:chrome
   firefox  = //java/client/src/org/openqa/selenium/firefox:firefox
-  htmlunit = //java/client/src/org/openqa/selenium/htmlunit:htmlunit
   remote   = //java/client/src/org/openqa/selenium/remote:remote
   safari   = //java/client/src/org/openqa/selenium/safari:safari