As now ::comphelper::existsValue is not used anymore, we're removing this too.

Change-Id: I9bd2544a9c378f5a18746255133f5684867e0114
Reviewed-on: https://gerrit.libreoffice.org/6378Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>