functions were declared twice, in stringio.hxx and poolio.hxx.

remove unicode functions as they were unused.
remove duplicate function declarations in poolio.hxx.
move implementations from poolio.cxx to new stringio.cxx.

Change-Id: I469346a01d793d22721e62bf7e96fd01e28d560c
Reviewed-on: https://gerrit.libreoffice.org/41331Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>