Remove middle name constraint following added option in ADCI/full-name-parser

Based on the many years of experience handling names and imports, we normally do not use many restrictions on names, especially not middle names and last name. The HumanNameParser already made the last name not mandatory, we propose to do the same on the middle name.

More can be read on https://github.com/ADCI/full-name-parser/pull/11 and https://github.com/ADCI/full-name-parser/pull/12