clippy: fix "this let-binding has unit value" warnings

[brooksprumo]

Problem

Clippy warns for let-bindings with unit values:

warning: this let-binding has unit value
   --> core/benches/banking_stage.rs:88:13
    |
88  | /             let _ignored = BankingStage::consume_buffered_packets(
89  | |                 &my_pubkey,
90  | |                 std::u128::MAX,
91  | |                 &poh_recorder,
...   |
100 | |                 10,
101 | |             );
    | |______________^
    |
    = note: `#[warn(clippy::let_unit_value)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
    |
88  ~             BankingStage::consume_buffered_packets(
89  +                 &my_pubkey,
90  +                 std::u128::MAX,
91  +                 &poh_recorder,
92  +                 &mut transaction_buffer,
93  +                 None,
  ...

Summary of Changes

Remove the let bindings.