gitaly: Remove Postgres client from Gitaly images

We're currently building Gitaly images with three different Postgres
versions. Ultimately though, Gitaly doesn't need Postgres installed in
its images at all: the Postgres server is tied in as a service, and the
only CI job that uses the the Postgres client is using a different image
anyway. One part that had been using the Postgres client in the past was
the logic to wait for Postgres to come up. But this has been migrated to
instead rely on the health-checking logic provided by CI services in
f89ffa3dc (ci: Stop waiting for Postgres manually, 2022-06-14)

Stop installing Postgres in Gitaly images to reduce the number of images
built.

.gitlab/ci/gitaly.images.yml

@@ -10,4 +10,3 @@ gitaly:
         RUBY: ['2.7', '3.0']
         GOLANG: ['1.17', '1.18']
         GIT: ['2.33']
-        POSTGRESQL: ['11', '12', '13'] # We only use the postgresql-client