hw/watchdog/wdt_i6300esb: remove a unnecessary comment

The registered memory region of i6300esb is not suitable for coalesced
mmio, because a write for the region may trigger an immediate action
and can't be delayed.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Message-Id: <1544253511-82742-1-git-send-email-peng.hao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>