DOC: remove unused north_vector in off_axis_projection example #8317

Job	Run time
unit tests: py3.9.2 on ubuntu-20.04 (pytest)	10m 42s
unit tests: py3.12 on ubuntu-latest (pytest)	15m 12s
unit tests: py3.12 on macos-latest (pytest)	11m 23s
answer tests: py3.9 on ubuntu-latest (nose)	14m 42s
unit tests: py3.9 on ubuntu-latest (nose)	14m 5s
unit tests: py3.x on ubuntu-latest (pytest)	9m 55s
unit tests: py3.12 on windows-latest (pytest)	17m 27s
Image tests	6m 7s
	1h 39m 33s

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