Trying to fix the problems I encountered earlier I found out that the removal of the OID security provider is performed by the following steps.
During the setup where OID is created as a security provider the ssoreg.sh is run on the OID layer. Remove this with the web-based ORASSO (http://host:port/pls/orasso).
But the second step involved the usage of the new osso_newsite.conf file. This is done with the $ORACLE_AS10131_HOME/Apache/Apache/bin/osso1013 . Just comment out the entries in the httpd.conf and the mod_osso.conf, create a new empty_osso.conf in $OH/Apache/Apache/conf/osso/ and rerun the osso1013.
This did the trick for me.