remove unused TextBufferDirection enum

BUG=skia:

Review URL: https://codereview.chromium.org/621403002
diff --git a/samplecode/SampleText.cpp b/samplecode/SampleText.cpp
index 17456b2..7f872a9 100644
--- a/samplecode/SampleText.cpp
+++ b/samplecode/SampleText.cpp
@@ -37,8 +37,7 @@
     SkScalar nn = 0;
     for (SkScalar w = 0; w <= width; w += SK_Scalar1) {
         SkScalar m;
-        size_t n = paint.breakText(text, length, w, &m,
-                                    SkPaint::kBackward_TextBufferDirection);
+        size_t n = paint.breakText(text, length, w, &m);
 
         SkASSERT(n <= length);
         SkASSERT(m <= width);