Only 1 trend line can be inserted. It is no more possible
to insert trend lines for all series at a time.
See comments in b0fbaa6f

Change-Id: I3a8548af3d3cd641b4da342ee03681647844fde3
Reviewed-on: https://gerrit.libreoffice.org/28226Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>