fwdanalysis: Remove unused What::ValueFlow mode #20021

Job	Run time
build_make (ubuntu:25.10)	2m 50s
build_cmake (ubuntu:25.10, true, true)	5m 21s
build_make (ubuntu:24.04)	2m 59s
build_cmake (alpine:3.23, false, false)	4m 23s
build_cmake (ubuntu:24.04, true, true)	2m 35s
build_make (alpine:3.23)	5m 46s
	23m 54s

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