Drop unused EmbeddedLink type

It generates this warning when building the docs:

```
WARNING: invalid signature for autoattribute ('gxformat2.model::EmbeddedLink.$link') [autodoc]
WARNING: don't know which module to import for autodocumenting 'gxformat2.model::EmbeddedLink.$link' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name) [autodoc]
```

gxformat2/model.py

@@ -10,19 +10,14 @@
     Union,
 )
 
-from typing_extensions import (
-    Literal,
-    TypedDict,
-)
+from typing_extensions import Literal
 
 log = logging.getLogger(__name__)
 
 DictOrList = Union[Dict, List]
 ConnectDict = dict
 
 
-EmbeddedLink = TypedDict("EmbeddedLink", {"$link": str})
-
 NativeGalaxyStepType = Literal[
     "subworkflow",
     "data_input",