Test: simplify 001.load_balance test.

Test: simplify 001.load_balance test.

Previously we checked the version of psql, so that we could adapt the
change in psql: i.e. SHOW_ALL_RESULTS addition.
By doing:
\set SHOW_ALL_RESULTS off
in 7.sql, the test results are now identical PostgreSQL 15 and before.
As a result, expected*-pre15 files were removed.