Remove unnecessary PGDATABASE settings from TAP tests

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