[ty] _typeshed.Self is not a special form (#18377)

## Summary

This change was based on a mis-reading of a comment in typeshed, and a
wrong assumption about what was causing a test failure in a prior PR.
Reverting it doesn't cause any tests to fail.

## Test Plan

Existing tests.

crates/ty_python_semantic/src/types/special_form.rs

@@ -79,7 +79,7 @@ pub enum SpecialFormType {
     /// The symbol `typing.Callable`
     /// (which can also be found as `typing_extensions.Callable` or as `collections.abc.Callable`)
     Callable,
-    /// The symbol `typing.Self` (which can also be found as `typing_extensions.Self` or `_typeshed.Self`)
+    /// The symbol `typing.Self` (which can also be found as `typing_extensions.Self`)
     #[strum(serialize = "Self")]
     TypingSelf,
     /// The symbol `typing.Final` (which can also be found as `typing_extensions.Final`)
@@ -227,16 +227,12 @@ impl SpecialFormType {
             | Self::TypeGuard
             | Self::TypedDict
             | Self::TypeIs
+            | Self::TypingSelf
             | Self::Protocol
             | Self::ReadOnly => {
                 matches!(module, KnownModule::Typing | KnownModule::TypingExtensions)
             }
 
-            Self::TypingSelf => matches!(
-                module,
-                KnownModule::Typing | KnownModule::TypingExtensions | KnownModule::Typeshed
-            ),
-
             Self::Unknown
             | Self::AlwaysTruthy
             | Self::AlwaysFalsy