Remove leftover `xcodeproj` folders #15

Job	Run time
build_and_test (04-tdd-in-the-real-world)	14m 18s
build_and_test (05-fixtures)	10m 6s
build_and_test (06-testing-static-swiftui-views)	10m 6s
build_and_test (07-testing-dynamic-swiftui-views)	9m 48s
build_and_test (08-stub)	9m 2s
build_and_test (09-json-decoding)	10m 18s
build_and_test (10-networking)	12m 25s
build_and_test (11-dependency-injection-with-environment-object)	17m 45s
build_and_test (12-spy)	19m 26s
build_and_test (13-testing-view-presentation)	11m 21s
build_and_test (14-fixing-bugs-and-changing-code)	11m 38s
build_and_test (15-fake-and-dummy)	12m 31s
build_and_test (17-appendix-b-nimble-only)	6m 11s
build_and_test (18-appendix-b-quick-and-nimble)	7m 2s
build_and_test (19-appendix-c-uikit)	7m 21s
	2h 49m 18s

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