packages: remove src from base set

The FreeBSD-src package is the largest single pkgbase package at 158M
currently. Remove it and FreeBSD-src-sys (87M) from the base set as
they are not useful to all users.

Reviewed by: emaste, ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52573

(cherry picked from commit 87a78d9618ab5430b12dd5c54ec61fcaeb8f0655)