[ty] Fix incorrect docstring in call signature completion (astral-sh#20021)

## Summary

This PR fixes astral-sh/ty#1071

The core issue is that `CallableType` is a salsa interned but
`Signature` (which `CallableType` stores) ignores the `Definition` in
its `Eq` and `Hash` implementation.

This PR tries to simplest fix by removing the custom `Eq` and `Hash`
implementation. The main downside of this fix is that it can increase
memory usage because `CallableType`s that are equal except for their
`Definition` are now interned separately.

The alternative is to remove `Definition` from `CallableType` and
instead, call `bindings` directly on the callee (call_expression.func).
However, this would require
addressing the TODO 

here
https://github.com/astral-sh/ruff/blob/39ee71c2a57bd6c51482430ba8bfe3728b4ca443/crates/ty_python_semantic/src/types.rs#L4582-L4586

This might probably be worth addressing anyway, but is the more involved
fix. That's why I opted for removing the custom `Eq` implementation.

We already "ignore" the definition during normalization, thank's to
Alex's work in astral-sh#19615

## Test Plan



https://github.com/user-attachments/assets/248d1cb1-12fd-4441-adab-b7e0866d23eb

Author: MichaReiser

crates/ty_python_semantic/src/types/signatures.rs

@@ -247,7 +247,7 @@ impl<'db> VarianceInferable<'db> for &CallableSignature<'db> {
 }
 
 /// The signature of one of the overloads of a callable.
-#[derive(Clone, Debug, salsa::Update, get_size2::GetSize)]
+#[derive(Clone, Debug, salsa::Update, get_size2::GetSize, PartialEq, Eq, Hash)]
 pub struct Signature<'db> {
     /// The generic context for this overload, if it is generic.
     pub(crate) generic_context: Option<GenericContext<'db>>,
@@ -993,28 +993,6 @@ impl<'db> Signature<'db> {
     }
 }
 
-// Manual implementations of PartialEq, Eq, and Hash that exclude the definition field
-// since the definition is not relevant for type equality/equivalence
-impl PartialEq for Signature<'_> {
-    fn eq(&self, other: &Self) -> bool {
-        self.generic_context == other.generic_context
-            && self.inherited_generic_context == other.inherited_generic_context
-            && self.parameters == other.parameters
-            && self.return_ty == other.return_ty
-    }
-}
-
-impl Eq for Signature<'_> {}
-
-impl std::hash::Hash for Signature<'_> {
-    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
-        self.generic_context.hash(state);
-        self.inherited_generic_context.hash(state);
-        self.parameters.hash(state);
-        self.return_ty.hash(state);
-    }
-}
-
 impl<'db> VarianceInferable<'db> for &Signature<'db> {
     fn variance_of(self, db: &'db dyn Db, typevar: BoundTypeVarInstance<'db>) -> TypeVarVariance {
         tracing::debug!(