posix: Deprecate custom removeAll/mkdirAll implementations. (#4808)

Since go1.8 os.RemoveAll and os.MkdirAll both support long
path names i.e UNC path on windows. The code we are carrying
was directly borrowed from `pkg/os` package and doesn't need
to be in our repo anymore. As a side affect this also
addresses our codecoverage issue.

Refer #4658

pkg/x/os/stat_windows_test.go

@@ -22,6 +22,7 @@ import (
 	"io/ioutil"
 	os1 "os"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"testing"
 	"unsafe"
@@ -207,3 +208,38 @@ func TestStatSymlinkLoop(t *testing.T) {
 		}
 	}
 }
+
+func TestFixLongPath(t *testing.T) {
+	// 248 is long enough to trigger the longer-than-248 checks in
+	// fixLongPath, but short enough not to make a path component
+	// longer than 255, which is illegal on Windows. (which
+	// doesn't really matter anyway, since this is purely a string
+	// function we're testing, and it's not actually being used to
+	// do a system call)
+	veryLong := "l" + strings.Repeat("o", 248) + "ng"
+	for _, test := range []struct{ in, want string }{
+		// Short; unchanged:
+		{`C:\short.txt`, `C:\short.txt`},
+		{`C:\`, `C:\`},
+		{`C:`, `C:`},
+		// The "long" substring is replaced by a looooooong
+		// string which triggers the rewriting. Except in the
+		// cases below where it doesn't.
+		{`C:\long\foo.txt`, `\\?\C:\long\foo.txt`},
+		{`C:/long/foo.txt`, `\\?\C:\long\foo.txt`},
+		{`C:\long\foo\\bar\.\baz\\`, `\\?\C:\long\foo\bar\baz`},
+		{`\\unc\path`, `\\unc\path`},
+		{`long.txt`, `long.txt`},
+		{`C:long.txt`, `C:long.txt`},
+		{`c:\long\..\bar\baz`, `c:\long\..\bar\baz`},
+		{`\\?\c:\long\foo.txt`, `\\?\c:\long\foo.txt`},
+		{`\\?\c:\long/foo.txt`, `\\?\c:\long/foo.txt`},
+	} {
+		in := strings.Replace(test.in, "long", veryLong, -1)
+		want := strings.Replace(test.want, "long", veryLong, -1)
+		if got := fixLongPath(in); got != want {
+			got = strings.Replace(got, veryLong, "long", -1)
+			t.Errorf("fixLongPath(%q) = %q; want %q", test.in, got, test.want)
+		}
+	}
+}