remove legacy moz flags

NOTRY=True
R=george@mozilla.com, scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/342803002
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index b131766..ed54a58 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -674,9 +674,7 @@
 
     [ 'skia_moz2d', {
       'defines': [
-        'SK_SUPPORT_LEGACY_LAYERRASTERIZER_API=1',
-        'SK_SUPPORT_LEGACY_COMPATIBLEDEVICE_CONFIG=1',
-        'SK_SUPPORT_LEGACY_GETTOTALCLIP=1',
+        # add flags here (e.g. SK_SUPPORT_LEGACY_...) needed by moz2d
       ],
     }],