Fix underline on active tab in Web

The underline in active tabs is being overridden in Web
by setting the bottom border to zero on the notebook. This was
added in commit f008bff348c22c1c87e45e4b3a5d0758e71b59b3.

I don't think this is needed anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=709284