fdo#40315 Improve text for moving average trendline

Change text in status bar for moving average trendline:
no equation, no R²

Change-Id: I0e18a72ff1f1d78e3bb28cab4d5f215c096898dc
Reviewed-on: https://gerrit.libreoffice.org/7015
Tested-by: Tomaž Vajngerl <quikee@gmail.com>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>