fix(src/main): There are unused variables and unused import elements

[xuesongbj]

There are unused variables and unused import elements.

$ make debug
cargo build
Compiling toda v0.2.0 (/home/toda)
warning: unused import: Comm
--> src/main.rs:43:29
|
43 | use jsonrpc::{start_server, Comm};
| ^^^^
|
= note: #[warn(unused_imports)] on by default

warning: unused variable: rx
--> src/main.rs:182:14
|
182 | let (tx, rx) = mpsc::channel();
| ^^ help: if this is intentional, prefix it with an underscore: _rx
|
= note: #[warn(unused_variables)] on by default

warning: 2 warnings emitted

Finished dev [unoptimized + debuginfo] target(s) in 14.78s