pkgbase: Make src package creation recipes more precise

Just remove the plist created by the respective rule. Otherwise the two
receipes can race with each other.

Fixes: d7d5c9efef03 ("pkgbase: Let source packages be built in parallel")
Reviewed by: bapt, emaste
Reported by: Mark Millard <marklmi@yahoo.com>
Differential Revision: https://reviews.freebsd.org/D46320