Remove tranparent paint hack from BackdropFilter (#29929)
Remove tranparent paint hack from BackdropFilter (#29929)

We can remove the hack because https://github.com/flutter/engine/pull/8183
has been rolled into the framework.
tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter