Remove io sample #6166

Job	Run time
ios-build	11m 42s
Test Flutter stable on ubuntu-latest	17m 29s
web/samples_index config check	43s
Test Flutter stable on macos-latest	16m 24s
Test Flutter stable on windows-latest	24m 59s
Test Flutter beta on ubuntu-latest	9m 57s
Test Flutter beta on macos-latest	15m 31s
Test Flutter beta on windows-latest	16m 27s
Test Flutter master on ubuntu-latest	10m 33s
Test Flutter master on macos-latest	15m 35s
Test Flutter master on windows-latest	18m 46s
	2h 38m 6s

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