Remove derive-helpers rule with unclear meaning

We're not certain what this rule means; we've run various tests and
could not confirm our interpretation of the rule.  E.g. (thanks to
ehuss for this):

```rust
use pm::{WithHelperAttr, helper};
#[derive(WithHelperAttr)]
#[helper]
#[derive(helper)] // ERROR: expected derive macro, found derive helper attribute `helper`
struct S;
```

Interestingly, this works and calls the attribute macro:

```rust
use pm::{WithHelperAttr, helper};
#[derive(WithHelperAttr, helper)]
#[helper]
struct S;
```

So that we can merge the rest, let's remove this rule for now and add
it back in future work.

Author: traviscross

src/names/name-resolution.md

@@ -350,9 +350,6 @@ The available scope kinds are visited in the following order. Each of these scop
 > [!EDITION-2018]
 > Starting in edition 2018 the `#[macro_use]` prelude is not visited when [`#[no_implicit_prelude]`][names.preludes.no_implicit_prelude] is present.
 
-r[names.resolution.expansion.macros.derive-helpers]
-Derive helper scopes are not visited when resolving derive macros in the parent scope (starting scope).
-
 r[names.resolution.expansion.macros.reserved-names]
 The names `cfg` and `cfg_attr` are reserved in the macro attribute [sub-namespace].