Remove unnecessary PGDATABASE settings from TAP tests
Some of the TAP tests have been historically setting the environment
variable PGDATABASE to 'postgres', which is not needed because
PostgreSQL::Test::Cluster already sets it when initialized. This commit
removes these explicit setups.
Note that the dependency of cluster -a with PGDATABASE (from
1caef31d9e55) is still documented.
Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACXLAz5dW3ZP+Fec8g6jQMMmDyCVT+qdbye2h7QJJmhsdw@mail.gmail.com