Because most of my customers do not have an e-mail address, I changed
Kernel/Config.pm such that the customer user list field parameters read:

       CustomerUserListFields => ['first_name', 'last_name'],

and commented out the 

<!-- dtl:block:CustomerSearchAutoCompleteFromInvalid -->

block in our custom template.  However, this resulted in OTRS refusing
to save any tickets until I restored the customer user list fields to:

       CustomerUserListFields => ['first_name', 'last_name', 'email'],

even though as I mentioned almost none of the customer users in my
environment have an e-mail address.  Is there a way to return to the
"Firstname, Last name" customer user list field set (discrding the
pointless e-mail field) and still save tickets in OTRS?