tun: delete tun + windows related tun code #204

Job	Run time
build (darwin, amd64)	1m 1s
build (darwin, arm64)	50s
build (android, arm64)	48s
build (linux, amd64)	45s
build (linux, arm64)	49s
build (linux, arm, 7)	43s
build (linux, riscv64)	49s
build (linux, mips64)	50s
build (linux, mips64le)	50s
build (linux, mips)	50s
build (linux, mipsle)	42s
build (linux, mips64, softfloat)	52s
build (linux, mips64le, softfloat)	50s
build (linux, mips, softfloat)	47s
build (linux, mipsle, softfloat)	45s
build (windows, amd64)	48s
build (windows, arm64)	45s
build (windows, 386)	52s
	14m 36s

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