Not all schema changes are allowed. See Disallowed schema changes for change restrictions.
Use either of the following methods to delete an attribute. The Web administration tool is the preferred method.
Web administration
If you have not done so already, expand Schema management in the navigation area, then click Manage attributes. To delete an attribute:
Command line
ldapmodify -D <admindn> -w <adminpw> -i myschemadelete.ldif
Where the myschemadelete.ldif file includes:
dn: cn=schema changetype: modify delete: attributetypes attributetypes: (<myAttribute-oid>)
See ldapmodify and ldapadd for more information about this command.