It could be because the installer had removed the sqlplus. Try running the installer again and try doing the uninstall, but make sure that you give the sys and the sys password correctly. Because, this is needed to remove the webdb schema on the database. Also make sure that you have the tns_admin=$ORACLE_HOME/network/admin setted up on your profile. This would help to pin point the location of the tnsnames.ora file location.

Hope this would help you.
Sam