[IPO] Remove an unused function declaration

The corresponding definition doesn't seem to have existed every since
the declaration was introduced on Jul 8, 2019 in commit
accd3e8.

FWIW, we do have AAReturnedValuesImpl::getAssumedUniqueReturnValue
declared and defined in Attributor.cpp.

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -3142,11 +3142,6 @@ struct AAReturnedValues
     : public IRAttribute<Attribute::Returned, AbstractAttribute> {
   AAReturnedValues(const IRPosition &IRP, Attributor &A) : IRAttribute(IRP) {}
 
-  /// Return an assumed unique return value if a single candidate is found. If
-  /// there cannot be one, return a nullptr. If it is not clear yet, return the
-  /// Optional::NoneType.
-  Optional<Value *> getAssumedUniqueReturnValue(Attributor &A) const;
-
   /// Check \p Pred on all returned values.
   ///
   /// This method will evaluate \p Pred on returned values and return