ci: remove `main_tests` from matrix #2661

Job	Run time
docs	18s
minimal	14s
msrv	1m 5s
quickchecking	15s
rustfmt-clippy	29s
test-expectations (ubuntu-latest)	45s
test-expectations (macos-latest)	1m 4s
check-cfg	23s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 46s
test-book	10s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 38s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 44s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 21s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 11s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 10s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 13s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	5m 56s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 19s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	4m 8s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	5m 19s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	4m 57s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	5m 0s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	8m 15s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	6m 1s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 22s
test (macos-latest, 16.0, 0, 0, 0)	1m 53s
success	1s
	1h 8m 37s

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