hw/timer/stm32f2xx_timer: Remove dead assignment

Fix warning reported by Clang static code analyzer:

    CC      hw/timer/stm32f2xx_timer.o
  hw/timer/stm32f2xx_timer.c:225:9: warning: Value stored to 'value' is never read
          value = timer_val;
          ^       ~~~~~~~~~

Reported-by: Clang Static Analyzer
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422133152.16770-9-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/timer/stm32f2xx_timer.c

@@ -222,7 +222,6 @@ static void stm32f2xx_timer_write(void *opaque, hwaddr offset,
     case TIM_PSC:
         timer_val = stm32f2xx_ns_to_ticks(s, now) - s->tick_offset;
         s->tim_psc = value & 0xFFFF;
-        value = timer_val;
         break;
     case TIM_CNT:
         timer_val = value;