🔥 Remove `flutter` as a submodule #7939

Job	Run time
test (Beta channel, beta, true)	4m 10s
test (Beta channel, beta, true)	4m 10s
test (Stable channel, stable, false)	20m 18s
test (Stable channel, stable, false)	23m 3s
deploy	0s
deploy	0s
	51m 41s

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