Remove _X_NONNULL macro

We could not find any evidence it was ever used, but defining it caused C++11 compilers to complain:

/usr/X11R6/include/X11/Xfuncproto.h:173:24: warning: ISO C does not permit named variadic macros [-Wvariadic-macros]
 #define _X_NONNULL(args...)  __attribute__((nonnull(args)))

See: https://patchwork.freedesktop.org/patch/158579/