move SkScalarMean into its only caller, reducing out public API exposure

include/core/SkScalar.h

Index: include/core/SkScalar.h
diff --git a/include/core/SkScalar.h b/include/core/SkScalar.h
index 4ab361604ff5260e8b2dcb2163c6bce2ccddd84a..10bfa7297c64d5ca65e11ca80559044343524de1 100644
--- a/include/core/SkScalar.h
+++ b/include/core/SkScalar.h
@@ -139,9 +139,6 @@ inline SkScalar SkScalarSquare(SkScalar x) { return x * x; }
 /** Returns the average of two SkScalars (a+b)/2
 */
 #define SkScalarAve(a, b)       (((a) + (b)) * 0.5f)
-/** Returns the geometric mean of two SkScalars
-*/
-#define SkScalarMean(a, b)      sk_float_sqrt((float)(a) * (b))
 /** Returns one half of the specified SkScalar
 */
 #define SkScalarHalf(a)         ((a) * 0.5f)