Completely remove the capsule and cross

[mohanson]

• Completely remove the capsule and cross.
• Upgraded clang to clang-18. Since capsule and its docker environment is removed, you must install clang-18 or riscv64-unknown-elf-gcc locally.

[XuJiandong]

fix warnings:

warning: unused variable: `err`
   --> /Users/xxx/projects/ckb-std/src/high_level.rs:569:17
    |
569 |             Err(err) => {
    |                 ^^^ help: if this is intentional, prefix it with an underscore: `_err`
    |
    = note: `#[warn(unused_variables)]` on by default

warning: `ckb-std` (lib) generated 1 warning (run `cargo fix --lib -p ckb-std` to apply 1 suggestion)
   Compiling simulator v0.1.0 (/Users/xxx/projects/ckb-std/test/simulator)
warning: unused variable: `v`
  --> src/../../../contracts/ckb-std-tests/src/entry.rs:34:9
   |
34 |     let v = vec![0u8; 42];
   |         ^ help: if this is intentional, prefix it with an underscore: `_v`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `data_hash`
   --> src/../../../contracts/ckb-std-tests/src/entry.rs:157:9
    |
157 |     let data_hash = packed::CellOutput::calc_data_hash(&data);
    |         ^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_data_hash`

warning: `simulator` (bin "contract") generated 2 warnings (run `cargo fix --bin "contract"` to apply 2 suggestions)
    Finished dev [unoptimized + debuginfo] target(s) in 1m 00s

[mohanson]

These warnings have been there since these PRs:

e544f02

22648e2