requirements-test.txt: Drop no longer needed pytest-postgresql requirement

requirements-swh.txt already declares the swh.core[db] dependency which
transitively pulls it.

Related to T2746

requirements-test.txt

 pytest
 pytest-mock
-pytest-postgresql >= 2.1.0
 celery >= 4.3
 hypothesis >= 3.11.0
 swh.lister

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T2746 is now swh/infra/sysadm-environment#2746 (closed)