Revert "mux: fix ctrl-a b again"

This reverts commit 1b2503fcf7.

Unfortunately this fix regresses console handling on MIPS Malta;
since the mux ctrl-a b bug is not a regression since 2.11, we
take the conservative approach and just drop it from 2.12.

Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

chardev/char-mux.c

@@ -304,7 +304,6 @@ void mux_set_focus(Chardev *chr, int focus)
     }
 
     d->focus = focus;
-    chr->be = d->backends[focus];
     mux_chr_send_event(d, d->focus, CHR_EVENT_MUX_IN);
 }