Don't restrict to 10.9/10.10 on darwin

We're trying to switch these to a minimum 10.13 builder, and
increasingly need to debug on modern versions, so it no longer makes
sense to have this restriction.

Test: Ran build-openjdk11-darwin.sh on Big Sur
Change-Id: I06d17fcaaac7d851883d6bfcfb017487ff3dd3b5