webadmin: Remove redundant header splitters in fixed columns

This was previously fixed in resizable columns, but not in fixed ones.

Change-Id: I0090109d3cc2c4a18418fef49eaf09b8a5c52915
Bug-Url: https://bugzilla.redhat.com/1139551
Signed-off-by: Lior Vernia <lvernia@redhat.com>