remove sk_mkdir: error 17 creating dir

As far as I've ever seen, this error only comes up when two threads or
processes race to create the same directory.  In that case, it's not
really an error... the directory is made either way.

I've never seen this message not just be spam to ignore.

Change-Id: I1afad5208fcb6b2ab2e94a1402ca7db0beb39817
Reviewed-on: https://skia-review.googlesource.com/47402
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/ports/SkOSFile_stdio.cpp

@@ -162,10 +162,5 @@ bool sk_mkdir(const char* path) {
 #else
     retval = mkdir(path, 0777);
 #endif
-    if (0 == retval) {
-        return true;
-    } else {
-        fprintf(stderr, "sk_mkdir: error %d creating dir '%s'\n", errno, path);
-        return false;
-    }
+    return 0 == retval;
 }