Remove Previous Version from .bs file

When publishing to WD the correct previous version gets added anyway, and having (an incorrect) previous version mentions in the .bs file actually breaks auto-publishing.

index.bs

@@ -11,7 +11,6 @@ Status: ED
 ED: https://w3c.github.io/FileAPI/
 TR: https://www.w3.org/TR/FileAPI/
 Repository: w3c/FileAPI
-Previous Version: https://www.w3.org/TR/2019/WD-FileAPI-20190531/
 !Tests: <a href=https://github.com/web-platform-tests/wpt/tree/master/FileAPI>web-platform-tests FileAPI/</a> (<a href=https://github.com/web-platform-tests/wpt/labels/FileAPI>ongoing work</a>)
 Abstract: This specification provides an API for representing file objects in web applications,
   as well as programmatically selecting them and accessing their data. This includes: