Remove postgres service from github action [Resolves #899]

[jtwalsh0]

testing.postgresql runs initdb locally. The postgres service doesn't get used.

[shaycrk]

Thanks @jtwalsh0!

@nanounanue and @KasunAmare -- how (if it all) does this interact with the work you've been doing to upgrade sqlalchemy and friends? Seems like if it goes unused there shouldn't be an issue, but I just wanted to run it by both of you before merging anything to be sure!