`org-export-export-preprocess-string' was a dummy copy of
`org-export-preprocess-string', adapted for specific needs
of org-mw-export. We now use `org-export-preprocess-string'
directly. This requires a minor change in this latter function:
if :for-backend is not explicitely passed, then the let binding
of `org-export-current-backend' falls back on the previous value
of org-export-current-backend'.
Bastien Guerry
Carsten Dominik