Remove duplicate postgres startup #11793

[OMERO-homebrew-develop](http://hudson.openmicroscopy.org.uk/job/OMERO-homebrew-develop) and [OMERO-homebrew-STABLE](http://hudson.openmicroscopy.org.uk/job/OMERO-homebrew-stable) regularly failed because Postgres was being started twice.

Note this requires changes to the jenkins job- since the postgres data dir is explicitly set it is necessary to run `/usr/local/bin/pg_ctl -D$PSQL_DIR -m fast stop` instead of `/usr/local/bin/brew services stop postgres`. Try runnning this job a few times to check it works.