DRY-cleanup RegressionCurveItemConverter

Change-Id: I8fb799b2efc81587252161ccadfa1dbd6b096bda
Reviewed-on: https://gerrit.libreoffice.org/6871
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>