ci: Remove extra PG_TEST_EXTRA from NetBSD/OpenBSD

The PG_TEST_EXTRA environment variable is already set at the top level.
As of 3d1aec225, Meson tasks will use this by default, so there's no
need for another intermediate variable.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/flat/CAN55FZ0aO8d_jkyRijcGP8qO%3DXH09qG%3Dpw0ZZDvB4LMzuXYU1w%40mail.gmail.com

.cirrus.tasks.yml

@@ -321,7 +321,6 @@ task:
         --pkg-config-path ${PKGCONFIG_PATH} \
         -Dcassert=true -Dinjection_points=true \
         -Dssl=openssl ${UUID} ${TCL} \
-        -DPG_TEST_EXTRA="$PG_TEST_EXTRA" \
         build
     EOF