Remove nightly CI jobs for distros that we don't support anymore #1048

Job	Run time
unit_test_execution (el/7)	5m 19s
unit_test_execution (el/8)	5m 38s
unit_test_execution (ol/7)	5m 50s
unit_test_execution (ol/8)	5m 54s
unit_test_execution (debian/stretch)	6m 5s
unit_test_execution (debian/buster)	6m 5s
unit_test_execution (debian/bullseye)	6m 30s
unit_test_execution (ubuntu/bionic)	5m 45s
unit_test_execution (ubuntu/focal)	6m 17s
	53m 23s

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