According to this you've done ok with the Trash. You can go through the list of folders it mentions as installation directories and remove them if still present:
- Binaries:
/Applications/Postgres.app/Contents/MacOS/bin
- Headers:
/Applications/Postgres.app/Contents/MacOS/include
- Libraries:
/Applications/Postgres.app/Contents/MacOS/lib
- Shared Libraries:
/Applications/Postgres.app/Contents/MacOS/share
- Data:
~/Library/Application\ Support/Postgres/var
Also you can check your psql installation running:
$ which psql
It should come from /usr/bin/
if it was installed properly by some source (macports, brew) and I presume the PostgresApp used its own bin folder for it.