Part revert #22 to undo memory fault (re: b7932e87)

The fix in sh/xec.c, which was backported from the ksh 93v- beta to
delay the actual removal of a running function that unsets itself,
caused a segfault in the variables.sh regression tests (see #23).

src/cmd/ksh93/sh/xec.c:
- Comment out the backported code pending a correct fix for #21.
  Now both types of functions silently fail to unset themselves
  (unless they're discipline functions).

src/cmd/ksh93/tests/functions.sh:
- Disable regression tests checking that the function was actually
  unset, pending a correct fix for #21.

Resolves: #23
Reopens: #21

src/cmd/ksh93/sh/xec.c

@@ -3505,11 +3505,16 @@ static void sh_funct(Shell_t *shp,Namval_t *np,int argn, char *argv[],struct arg
 	nv_putval(SH_PATHNAMENOD,shp->st.filename,NV_NOFREE);
 	shp->pipepid = pipepid;
 	np->nvalue.rp->running  -= 2;
+/*
+ * TODO: fix https://github.com/ksh93/ksh/issues/21
+ * Following causes a memory fault in: bin/shtests -p variables
+ *
 	if(np->nvalue.rp && np->nvalue.rp->running==1)
 	{
 		np->nvalue.rp->running = 0;
 		_nv_unset(np, NV_RDONLY);
 	}
+*/
 }
 
 /*