Revert "hw/char/pl011: Warn when using disabled receiver"

Revert "hw/char/pl011: Warn when using disabled receiver"

The guest does not control whether characters are sent on the UART.
Sending them before the guest happens to boot will now result in a
"guest error" log entry that is only because of timing, even if the
guest _would_ later setup the receiver correctly.

This reverts the bulk of commit abf2b6a028670bd2890bb3aee7e103fe53e4b0df,
and instead adds a comment about why we don't check the enable bits.

Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20250311153717.206129-1-pbonzini@redhat.com
[PMM: expanded comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>