ephy-location-entry: no need to set icons activatable

Icons are activatable by default in GtkEntry. No need to reiterate that.

https://bugzilla.gnome.org/show_bug.cgi?id=669129
ephy-location-entry: disable icon drag for blank pages

If the address being set by ephy_location_entry_set_location is NULL or
about:blank, the entry will disable dragging for the
GTK_ENTRY_ICON_PRIMARY icon.

It is enabled by default on widget construction.

https://bugzilla.gnome.org/show_bug.cgi?id=669129