Delete `buildosx` from project root #1751

Job	Run time
ubuntu (ubuntu-latest, 3.10, tests/model)	11m 39s
windows (windows-latest, 3.12, tests/model)	17m 51s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_statespace.py)	9m 12s
windows (windows-latest, 3.12, tests/statespace/test_statespace.py)	15m 17s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_kalman_filter.py)	3m 25s
ubuntu (ubuntu-latest, 3.10, tests/statespace --ignore tests/statespace/test_statespace.py --igno...	9m 8s
windows (windows-latest, 3.12, tests/statespace/test_kalman_filter.py)	8m 20s
ubuntu (ubuntu-latest, 3.10, tests/distributions)	7m 56s
windows (windows-latest, 3.12, tests/statespace --ignore tests/statespace/test_statespace.py --ig...	14m 59s
ubuntu (ubuntu-latest, 3.10, tests --ignore tests/model --ignore tests/statespace --ignore tests/...	14m 54s
windows (windows-latest, 3.12, tests/distributions)	15m 29s
windows (windows-latest, 3.12, tests --ignore tests/model --ignore tests/statespace --ignore test...	25m 14s
all_tests	1s
	2h 33m 25s

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