hw/char: sifive_uart: Remove outdated comment about Tx FIFO

Since Tx FIFO is now implemented using "qemu/fifo8.h", remove the comment
that no longer reflects the current implementation.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250911160647.5710-4-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>