webadmin: Remove redundant header splitter

The styling of ResizableHeader added a right border, which is
redundant in the case of an EntityModelCellTable because there's
always the left border of the next column header (as opposed to
ActionCellTable whose styling works differently).

Change-Id: Ied14df6f093514e26bbeb3dc60e509d58d056dae
Bug-Url: https://bugzilla.redhat.com/1125192
Signed-off-by: Lior Vernia <lvernia@redhat.com>