curlx: drop unused curlx_saferealloc()

Unused since 67ae101666 #19949

Closes #20504

lib/curlx/dynbuf.c

@@ -102,8 +102,6 @@ static CURLcode dyn_nappend(struct dynbuf *s,
   }
 
   if(a != s->allc) {
-    /* this logic is not using curlx_saferealloc() to make the tool not have to
-       include that as well when it uses this code */
     void *p = curlx_realloc(s->bufr, a);
     if(!p) {
       curlx_dyn_free(s);