Remove mbstowcs from <wchar.h>.

Glibc doesn't export the function here, and adding a <bits/...> header
for this function seems unnecessary.

Bug: http://b/28067717
Change-Id: I7304cc1daca14585c85d5c783365c4a43779ff2c
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index 0a94cee..fe068be 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -97,7 +97,6 @@
 extern size_t            mbrtowc(wchar_t *, const char *, size_t, mbstate_t *);
 extern size_t mbsrtowcs(wchar_t*, const char**, size_t, mbstate_t*);
 extern size_t mbsnrtowcs(wchar_t*, const char**, size_t, size_t, mbstate_t*);
-extern size_t            mbstowcs(wchar_t *, const char *, size_t);
 extern wint_t            putwc(wchar_t, FILE *);
 extern wint_t            putwchar(wchar_t);
 extern int               swprintf(wchar_t *, size_t, const wchar_t *, ...);