Simplify pg_isready usage

The pg_isready documentation says that it does not need a correct
database name or username to get the server status. In fact, incorrect
values result in the server logging failed connection attempts. As a
result, when we set DB_NAME to ALL, calls to the check_availability
function (which uses pg_isready) cause the server to log the following
error:

    FATAL:  database "ALL" does not exist

To eliminate this error, this change simplifies the pg_isready call.

install/assets/functions/10-db-backup

@@ -437,8 +437,7 @@ check_availability() {
             ;;
             "pgsql" )
                 counter=0
-                export PGPASSWORD=${DB_PASS}
-                until pg_isready --dbname=${DB_NAME} --host=${DB_HOST} --port=${DB_PORT} --username=${DB_USER} -q
+                until pg_isready --host=${DB_HOST} --port=${DB_PORT} -q
                 do
                     sleep 5
                     (( counter+=5 ))