Rollup merge of #66344 - petrochenkov:noregattr, r=matthewjasper

rustc_plugin: Remove `Registry::register_attribute`

Legacy plugins cannot register inert attributes anymore.

The preferred replacement is to use `register_tool` ([tracking issue](https://github.com/rust-lang/rust/issues/66079)).
```rust
#![register_tool(servo)]

#[servo::must_root]
struct S;
```

The more direct replacement is `register_attribute` ([tracking issue](https://github.com/rust-lang/rust/issues/66080))
```rust
#![register_attr(must_root)]

#[must_root]
struct S;
```
, but it requires registering each attribute individually rather than registering the tool once, and is more likely to be removed rather than stabilized.

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