etc/mtree/BSD.include.dist: Remove libmilter-dev

When building with WITHOUT_SENDMAIL this result in a FreeBSD-libmilter-dev
package with only this directory and a dependacy on FreeBSD-libmilter which
doesn't exists.

Differential Revision: https://reviews.freebsd.org/D54193
Fixes: 436618a427b4 ("etc/mtree: Add package tags for /usr/include")
Reviewed by: ivy
Sponsored by: Beckhoff Automation GMbH & Co. KG