os.h: drop obsolete xallocarray()

This macro isn't used anywhere (also not in drivers), so no need
to keep it around any longer.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/os.h

@@ -77,8 +77,6 @@ typedef struct _NewClientRec *NewClientPtr;
 
 #define xstrdup(s) Xstrdup(s)
 #define xnfstrdup(s) XNFstrdup(s)
-
-#define xallocarray(num, size) reallocarray(NULL, (num), (size))
 #endif
 
 #include <stdio.h>