pkgbase: Make src package creation recipes more precise

[markj]

Summary

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")
Reported by: Mark Millard <[e-mail]>

[emaste]

|| : seems unnecessarily clever to me, IMO || true is more obvious
but what error are we trying to ignore here anyway?

[markj]

In D46320#1056385, @emaste wrote:

|| : seems unnecessarily clever to me, IMO || true is more obvious
but what error are we trying to ignore here anyway?

Yeah, I'm not sure. -f should ensure that the exit status is 0 if the file doesn't exit.