Remove a now unneeded cast (xub_StrLen and tools/string.hxx removal).

Change-Id: Ic45b3c572483aa0292c227274565a1750dd51f17
Reviewed-on: https://gerrit.libreoffice.org/7854
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>