Remove obsolete libffi

Change-Id: I4fe528c90593a891456fda74bd22b0da1c549b0f
diff --git a/default.xml b/default.xml
index 81ffae0..555c88a 100644
--- a/default.xml
+++ b/default.xml
@@ -159,7 +159,6 @@
   <project path="external/libcxx" name="platform/external/libcxx" />
   <project path="external/libcxxabi" name="platform/external/libcxxabi" />
   <project path="external/libexif" name="platform/external/libexif" />
-  <project path="external/libffi" name="platform/external/libffi" />
   <project path="external/libgsm" name="platform/external/libgsm" groups="pdk" />
   <project path="external/liblzf" name="platform/external/liblzf" groups="pdk" />
   <project path="external/libmtp" name="platform/external/libmtp" />