remove alphatype from colortable

the owning bitmap is (already) responsible for knowing the alphatype

BUG=skia:
R=djsollen@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/611093002
diff --git a/src/images/SkImageDecoder_wbmp.cpp b/src/images/SkImageDecoder_wbmp.cpp
index ddb7c3d..0bf1389 100644
--- a/src/images/SkImageDecoder_wbmp.cpp
+++ b/src/images/SkImageDecoder_wbmp.cpp
@@ -119,7 +119,7 @@
     }
 
     const SkPMColor colors[] = { SK_ColorBLACK, SK_ColorWHITE };
-    SkColorTable* ct = SkNEW_ARGS(SkColorTable, (colors, 2, kOpaque_SkAlphaType));
+    SkColorTable* ct = SkNEW_ARGS(SkColorTable, (colors, 2));
     SkAutoUnref   aur(ct);
 
     if (!this->allocPixelRef(decodedBitmap, ct)) {