Remove mbtowc stub.

Change-Id: I63684e162883733f239ed83d202520e0f4be7dc5
diff --git a/src/stubs.cpp b/src/stubs.cpp
index d8b1ecb..1e20ada 100644
--- a/src/stubs.cpp
+++ b/src/stubs.cpp
@@ -70,12 +70,6 @@
     return (size_t)-1;
 }
 
-int mbtowc(wchar_t *, const char *, size_t)
-{
-    UNIMPLEMENTED;
-    return -1;
-}
-
 nl_catd catopen(const char *, int)
 {
     UNIMPLEMENTED;