chore(net): use smallvec only if debug_assertions configed

[jsvisa]

make build happy without warning

warning: unused imports: `SmallVec`, `smallvec`
  --> crates/net/network/src/transactions/fetcher.rs:20:16
   |
20 | use smallvec::{smallvec, SmallVec};
   |                ^^^^^^^^  ^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: unused variable: `peer_id`
    --> crates/net/network/src/transactions/fetcher.rs:1114:9
     |
1114 |         peer_id: &PeerId,
     |         ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_peer_id`
     |
     = note: `#[warn(unused_variables)]` on by default

[emhane]

didn't know that variables _var could still be used, nice! https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=383517bfe2a99eb05c474d2d1ba6f684

[emhane]

fyi @jsvisa there is an issue for building release in ci #6660

[jsvisa]

fyi @jsvisa there is an issue for building release in ci #6660

thanks for the info, but I didn't get the point of the link, do you want to link to #6434?

BTW, seems the debug_assertions config was not available via CI or makefile, or is there anything I'm missing.