[cli] remove invalid log when tx buffer is full (#13091)

In CliUartOutput, if otPlatUartFlush() fails when trying to send
buffered output to make room for new output, it logs a warning using
otLogWarnPlat. However, this warning is added to the same full
buffer, which does not help and can cause further issues.

This commit removes the offending log line as suggested in issue #7478.

examples/apps/cli/cli_uart.cpp

@@ -342,7 +342,6 @@ static int CliUartOutput(void *aContext, const char *aFormat, va_list aArguments
                 else
                 {
                     // Flush did not succeed, so abandon buffered output.
-                    otLogWarnPlat("Failed to output CLI: %s", otThreadErrorToString(error));
                     break;
                 }
             }