remove unused code (ancient Calc function ZELLERROR)

code has no function, except that calc documents with formula
ZELLERROR will not produce an error. But the formula is not
interpreted, so remains 'dead code'.

Change-Id: I6e7df7040a0273bcd3ebd497055924b7afdd3311
Reviewed-on: https://gerrit.libreoffice.org/7018
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>