message: Remove incorrect assertion

It was there only to prove that *now* `len` argument is not used to forbid 
putting message into history (i.e. that Neovim behaviour did not change). After 
this commit it should be possible to use msg_puts_attr_len with len>=0 to put 
message into history should new code need this.

src/nvim/message.c

@@ -1581,7 +1581,6 @@ void msg_puts_attr_len(const char *const str, const ptrdiff_t len, int attr)
 
   // if MSG_HIST flag set, add message to history
   if (attr & MSG_HIST) {
-    assert(len < 0);
     add_msg_hist(str, (int)len, attr);
     attr &= ~MSG_HIST;
   }