Since 4.2.8/4.3.2 isc-dhcp-server started to check if prefix6 range is
within declared subnet and if prefix6 network mask is bigger or equal
subnet mask. Remove these checks and make prefix delegation work as
expected.

For reference - https://redmine.pfsense.org/issues/4829

PR: 201601
Approved by: zi (maintainer)
Sponsored by: Netgate