you've to seperate 2 things

1. delegate in ldap allows user X to send emails in behalf of person Y (this can be done using kolab-webadmin)
2. sharing folders in imap allows user X to have access (Read, etc) of user Y (this can be done using folder management in roundcube, kolab cli or vanilla cyradm or imap protocol)

If you want to combine both features (Allow X to send behalf of Y and give folder rights from X to Y) then you can do both things at once using the kolab_delegate plugin.

As kolab-webadmin is basically only a ldap editor then you won't remove this feature.

If you still feel the need that you don't want to see this field/attribute in kolab-webadmin then please change the user_type formular as Directory Manager

IMO this this is not a bug

Yes, I agree it's not a software bug, but I propose to add some documentation about delegation to
https://docs.kolab.org/administrator-guide/using-the-kolab-web-administration-panel.html#users-administration
[which is empty right now]. Something like:

[contact information]
external email address: Allows the user to send mail with this foreign mail address

[configuration]
delegates: enables to delegatees to send mail on behalf of this user. This delegation
is stored in the LDAP server.
The delegates won't have access to the inbox or calender of the user. You need to
enable that in the webmail frontend (settings -> delegation). Access control is handled
by cyrus-imapd.