webadmin,userportal: Fix FindBugs warning in AbstractActionTable

FindBugs reports a false-positive "impossible cast" warning
in AbstractActionTable.getTableHeaderCells() method, since
CellTable.getElement() method always returns TableElement at
runtime.

This patch fixes the above mentioned warning.

Change-Id: Id53a2306d8d737c42f24d9093899a764319b0c2b
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>