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

 hypothesis >= 3.11.0
 pytest
 pytest-mock
-pytest-postgresql >= 2.1.0
 sqlalchemy-stubs
 # pytz is in fact a dep of swh.model[testing] and should not be necessary, but
 # the dep on swh.model in the main requirements-swh.txt file shadows this one

[Phabricator Migration user]

Some references in the commit message have been migrated:

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