Remove OS-specific versions of fs.MkdirAll

Go has supported Windows paths correctly since 1.11, see
https://github.com/golang/go/issues/10900 and the commit referenced
there.

internal/fs/file_unix.go

@@ -14,14 +14,6 @@ func fixpath(name string) string {
 	return name
 }
 
-// MkdirAll creates a directory named path, along with any necessary parents,
-// and returns nil, or else returns an error. The permission bits perm are used
-// for all directories that MkdirAll creates. If path is already a directory,
-// MkdirAll does nothing and returns nil.
-func MkdirAll(path string, perm os.FileMode) error {
-	return os.MkdirAll(fixpath(path), perm)
-}
-
 // TempFile creates a temporary file which has already been deleted (on
 // supported platforms)
 func TempFile(dir, prefix string) (f *os.File, err error) {