release/ec2: Remove references to portsnap

We no longer install a default portsnap.conf, so the sed invocation just
generates an error.

Reviewed by: cperciva
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42003

(cherry picked from commit 63aa2e91342ead0495eada5667f413f0cf8eb56a)