drop subtitle

As ReSpec now recognises the Living Document status and correctly
marks it as "W3C Living Document", we can drop our own custom
subtitle.

Author: andreastt

index.html

@@ -11,7 +11,6 @@
   specStatus: "LD",
   prEnd: "2018-05-24",
   shortName: "webdriver",
-  subtitle:  "Living Document",
   // publishDate: "2009-08-06",
   // copyrightStart: "2005"
   previousPublishDate: "2018-05-24",