Rollup merge of #93753 - jeremyBanks:main-conflict, r=petrochenkov

Complete removal of #[main] attribute from compiler

resolves #93786

---

The `#[main]` attribute was mostly removed from the language in #84217, but not completely. It is still recognized as a builtin attribute by the compiler, but it has no effect. However, this no-op attribute is no longer gated by `#[feature(main)]` (which no longer exists), so it's possible to include it in code *on stable* without any errors, which seems unintentional. For example, the following code is accepted ([playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&code=%23%5Bmain%5D%0Afn%20main()%20%7B%0A%20%20%20%20println!(%22hello%20world%22)%3B%0A%7D%0A)).

```rust
#[main]
fn main() {
    println!("hello world");
}
```

Aside from that oddity, the existence of this attribute causes code like the following to fail ([playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&code=use%20tokio%3A%3Amain%3B%0A%0A%23%5Bmain%5D%0Afn%20main()%20%7B%0A%20%20%20%20println!(%22hello%20world%22)%3B%0A%7D%0A)). According https://github.com/rust-lang/rust/pull/84062#issuecomment-825038275, the removal of `#[main]` was expected to eliminate this conflict (previously reported as #62127).

```rust
use tokio::main;

#[main]
fn main() {
    println!("hello world");
}
```

```
error[E0659]: `main` is ambiguous
 --> src/main.rs:3:3
  |
3 | #[main]
  |   ^^^^ ambiguous name
  |
  = note: ambiguous because of a name conflict with a builtin attribute
  = note: `main` could refer to a built-in attribute
```

[This error message can be confusing](https://stackoverflow.com/q/71024443/1114), as the mostly-removed `#[main]` attribute is not mentioned in any documentation.

Since the current availability of `#[main]` on stable seems unintentional, and to needlessly block use of the `main` identifier in the attribute namespace, this PR finishes removing the `#[main]` attribute as described in https://github.com/rust-lang/rust/issues/29634#issuecomment-274951753 by deleting it from `builtin_attrs.rs`, and adds two test cases to ensure that the attribute is no longer accepted and no longer conflicts with other attributes imported as `main`.

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