inline RegexpMapIterImpl into RegexpMapConstIter

no need for header only classes to have an Impl

Change-Id: I66236e3f9137320cfe39d4312cf7682c1d808815
Reviewed-on: https://gerrit.libreoffice.org/58561
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>