Bug#858839: webkit2gtk: Don't recommend gtk2 library

[Jeremy Bicha]

Package: libwebkit2gtk-4.0-37
Version: 2.10.4+dfsg1-1

Please don't have libwebkit2gtk-4.0-37 recommend the gtk2 package.

I think many apps that use webkit don't need Flash support. Those that
do use Flash can recommend or suggest the gtk2 package themselves.

I'm interested in trying to make it easier for users of the next
Ubuntu and Debian (buster) to not have gtk2 installed.
https://launchpad.net/bugs/1585903

Thanks,
Jeremy Bicha