ci: Remove ulimit -p for netbsd/openbsd

Previously the ulimit -p 256 was needed to increase the limit on
openbsd. However, sometimes the limit actually was too low, causing
  "could not fork new process for connection: Resource temporarily unavailable"
errors.  Most commonly on netbsd, but also on openbsd.

The ulimit on openbsd couldn't trivially be increased with ulimit, because of
hitting the hard limit.

Instead of increasing the limit in the CI script, the CI image generation now
increases the limits: https://github.com/anarazel/pg-vm-images/pull/129

Backpatch-through: 18

.cirrus.tasks.yml

@@ -384,9 +384,6 @@ task:
     su postgres <<-EOF
       set -e
       ulimit -c unlimited
-      # Otherwise tests will fail on OpenBSD, due to inability to start enough
-      # processes.
-      ulimit -p 256
       meson test $MTEST_ARGS --num-processes ${TEST_JOBS}
     EOF