Don't generate #[main] and #![feature(main)] as it was removed

[kkysen]

When the C main function is not int main() (such as void main()), c2rust transpile marks the transpiled fn main as #[main] and adds #![feature(main)] to the file. However, #[main] and #![feature(main)] have been removed in Rust for a while now since 2017 (in rust-lang/rust#29634) and are now both errors. We should stop emitting both of these. Given Rust no longer supports it, I don't think we should try to support non-standard main functions in C, which are implementation-defined anyways. In the specific case of void main() from #661, as well as other signatures that return void instead of int, we could still support them by just returning () from fn main.

[bytewife]

Hey, I am re-running the culprit code in #661 and it appears that it transpiles main.c fine, and rustc compiles and runs main.rs as expected. I'm also unable to find where #[main] might be generated in /c2rust-transpile, so I believe this issue is no longer valid.
main.c:

#include <stdio.h>
void main() {
     // printf() displays the string inside quotation
     printf("Hello, World!");

}

main.rs:

#![allow(dead_code, mutable_transmutes, non_camel_case_types, non_snake_case, non_upper_case_globals, unused_assignments, unused_mut)]
extern "C" {
    fn printf(_: *const libc::c_char, _: ...) -> libc::c_int;
}
unsafe fn main_0() {
    printf(b"Hello, World!\0" as *const u8 as *const libc::c_char);
}
pub fn main() {
    unsafe { main_0() }
    ::std::process::exit(0i32);
}

ivy:~/projects/c2rust_test_c is 📦 v0.1.0 via C v14.0.0-clang via △ v3.25.1 via 🦀 v1.68.0-nightly via 🐏 9GiB/16GiB took 24s450ms  cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/test`
Hello, World!%   

[kkysen]

Thanks for checking this. #661 must've been using an old version of c2rust transpile, and I didn't check myself.

#[main] was removed in 2e4ef2a.