Remove postgres from K8S venv (apache#28323)

While postgres has always been in the k8s venv used for K8S tests,
it seems it is not needed there at all. Not sure if it was
ever needed in fact (maybe some super old historical version of
those tests needed it). This removes postgres from the requirements
for the venv.

scripts/ci/kubernetes/k8s_requirements.txt

@@ -1,7 +1,6 @@
-.[cncf.kubernetes,postgres]
+.[cncf.kubernetes]
 pytest
 pytest-cov
 pytest-instafail
 pytest-timeouts
 pytest-xdist
-freezegun