Remove misleading SKIP_GCCPP_DEFINITIONS from macros documentation

(fix of commit 6e110ee)

PR #643 (bdwgc).

Instead of prohibiting of new and delete operators definition in
gc_cpp.cc, it would be better for the client to link with libgctba.a
(not with libgccpp.a).

* docs/README.macros (SKIP_GCCPP_DEFINITIONS): Remove.

docs/README.macros

@@ -268,10 +268,6 @@ REDIRECT_REALLOC=<X>    Causes realloc to be redirected to X.
 REDIRECT_FREE=<X>       Causes free to be redirected to X.  The canonical use
   is REDIRECT_FREE=GC_debug_free.
 
-SKIP_GCCPP_DEFINITIONS  Do not define new and delete operators in gc_cpp.cc.
-  This might be useful for compiling and linking of client code with
-  user-supplied allocation definitions.
-
 IGNORE_FREE     Turns calls to free into a no-op.  Only useful with
   REDIRECT_MALLOC.