YJIT: Remove redundant `extern crate` (#5869)

Follow up https://github.com/ruby/ruby/commit/0514d81

Rust YJIT requires Rust 1.60.0 or later. So, `extern crate` looks unnecessary
because it can use the following Rust 2018 edition feature:
https://doc.rust-lang.org/stable/edition-guide/rust-2018/path-changes.html#no-more-extern-crate

It passes the following tests.

```console
% cd yjit
% cargo test --features asm_comments,disasm
(snip)

test result: ok. 56 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```

yjit/src/asm/x86_64/tests.rs

@@ -404,7 +404,6 @@ fn test_xor() {
 #[cfg(feature = "disasm")]
 fn basic_capstone_usage() -> std::result::Result<(), capstone::Error> {
     // Test drive Capstone with simple input
-    extern crate capstone;
     use capstone::prelude::*;
     let cs = Capstone::new()
         .x86()

yjit/src/disasm.rs

@@ -66,7 +66,6 @@ fn disasm_iseq(iseq: IseqPtr) -> String {
     }
 
     // Initialize capstone
-    extern crate capstone;
     use capstone::prelude::*;
     let cs = Capstone::new()
         .x86()