etc/mtree/BSD.include.dist: Remove bsnmp tags

When building with WITHOUT_BSNMP this result in a FreeBSD-bsnmp-dev
package with only this directory and a dependency on FreeBSD-bsnmp which
doesn't exists.

Reviewed by: ivy
Differential Revision: https://reviews.freebsd.org/D54235
Fixes: 436618a427b4 ("etc/mtree: Add package tags for /usr/include")
Sponsored by: Beckhoff Automation GMbH & Co. KG