hw/rtc/mc146818rtc: Drop pre-v3 migration stream support

hw/rtc/mc146818rtc: Drop pre-v3 migration stream support

mc146818rtc's migration stream is at version 3 since commit
56038ef6234 ("RTC: Update the RTC clock only when reading it")
from 12 years ago, released in QEMU v1.3.0!
No versioned machines are that old, we can safely remove
support for older streams and the qdev_set_legacy_instance_id()
call.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250115210048.25396-1-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>