pkg/machine/shim: fix unused warnings on windows

These:

> pkg/machine/shim/claim_unsupported.go:9:6: func dockerClaimHelperInstalled is unused (unused)
> func dockerClaimHelperInstalled() bool {
>      ^
> pkg/machine/shim/claim_unsupported.go:13:6: func claimDockerSock is unused (unused)
> func claimDockerSock() bool {
>      ^
> pkg/machine/shim/claim_unsupported.go:17:6: func dockerClaimSupported is unused (unused)
> func dockerClaimSupported() bool {
>      ^

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/machine/shim/claim_unsupported.go

@@ -5,15 +5,3 @@ package shim
 func findClaimHelper() string {
 	return ""
 }
-
-func dockerClaimHelperInstalled() bool {
-	return false
-}
-
-func claimDockerSock() bool {
-	return false
-}
-
-func dockerClaimSupported() bool {
-	return false
-}

pkg/machine/shim/claim_unsupported_unix.go

@@ -0,0 +1,15 @@
+//go:build unix && !darwin
+
+package shim
+
+func dockerClaimHelperInstalled() bool {
+	return false
+}
+
+func claimDockerSock() bool {
+	return false
+}
+
+func dockerClaimSupported() bool {
+	return false
+}