Remove outdated `Cargo.toml` comment about `declare_interior_mutable_const` (#44950)

Bertie690 created 

Since the rule is no longer a `style` lint as of
[mid-August](https://github.com/rust-lang/rust-clippy/pull/15454), the
comment mentioning it not being one is outdated and should be removed.

> [!NOTE]
> I kept the severity at `error` for now to avoid rustling feathers.
> If `warn` is preferred, feel free to change it yourself or ask me to
do it - it's only 1 line of code, after all.

Release Notes:

- N/A

Change summary

Cargo.toml | 2 --
1 file changed, 2 deletions(-)

Detailed changes

Cargo.toml 🔗 

@@ -857,8 +857,6 @@ unexpected_cfgs = { level = "allow" }
 dbg_macro = "deny"
 todo = "deny"
 
-# This is not a style lint, see https://github.com/rust-lang/rust-clippy/pull/15454
-# Remove when the lint gets promoted to `suspicious`.
 declare_interior_mutable_const = "deny"
 
 redundant_clone = "deny"