nautilus: Remove duplicate borders

The toolbar has a bottom border now, which, added to the top borders on the places sidebar, the pane separator, and the notebook frame, results in a double border. Remove the later three to get a single border.

https://bugzilla.gnome.org/show_bug.cgi?id=706673
nautilus: Remove search bar top border, add bottom border

The search bar theming was expecting a border below and no border above. Now
that the toolbar is drawing the border, we need to draw a bottom border instead
of a top border.

https://bugzilla.gnome.org/show_bug.cgi?id=706673
nautilus: Drop search bar overrides for tabs

The theming of the search bar for "no tabs" is identical now.

https://bugzilla.gnome.org/show_bug.cgi?id=706673
nautilus: simplify CSS a bit