I guess you have an Access Role in your database which still references to this AD group object while this AD group object references to the LDAP Account Unit you want to delete. Use CPMI (GuiDBedit for example) to search for this object (ad_group_IT_Users).
When you found the Access Role(s) which use(s) this ad group object, remove the ad group from this Access Role object(s) using SmartConsole. After that (at least after publish and install database), try to remove the LDAP Account unit again.
If it is still saying it is in use, then use GuiDBedit to search for other references for the LDAP Account Unit, SmartConsole is not able to find with its where-used feature.
The main problem here it, that SmartConsoles where-used feature cannot display all references of all objects. CPMI usually can.