Revert "Reapply "release.sh: Add -jN to make release""

Release builds on 14.x have been failing for unclear reasons; the most
likely culprit is the recently-enabled parallel release building. Turn
it off on the 14.1 release branch to make sure we don't run into any
problems with building the release.

This is a direct commit to releng/14.1 -- I'm leaving parallel release
building turned on in stable/14 so I can try to track down the problems
there (and it's also enabled in HEAD, where I haven't seen any failures).

This reverts commit db34bbcedbf5b9fe7c015856cd04136923e1ce06.

Discussed with: karels
Approved by: re (cperciva)