remove_message does not store any pointer to x0

On OTP 20 erlang compiler generates following code:
```
{get_tuple_element,{x,0},1,{x,0}}.
remove_message.
```

x0 is clearly still used and it shouldn't be overwritten by
remove_message, also in msg_instrs.tab only loop_rec has
`r(0) = ERL_MESSAGE_TERM(msgp);`.

chapters/beam_instructions.asciidoc

@@ -346,8 +346,8 @@ handling.
 
 For code like +receive _ -> ok end+, where we accept any messages,
 there is no pattern matching needed, we just do a +remove_message+
-which unlinks the next message from the message queue and stores a
-pointer in X0. (It also removes any timeout, more on this soon.)
+which unlinks the next message from the message queue. (It also
+removes any timeout, more on this soon.)
 
 ==== A Selective Receive Loop