Moves the custom cleanup logic to overridden SbxMethod::Clear, to simplify
the cleanup code and make sure it restores empty Variant correctly.
Change-Id: I01fa0529acd9ac787ffcda60fd6836ade4afdcb1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/136108
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>