Remove redundant schema_path variable

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

altair/utils/schemapi.py

@@ -242,13 +242,8 @@ def __str__(self):
         )
 
         # Output all existing parameters when an unknown parameter is specified
-        if (
-            hasattr(vegalite, schema_path.split(".")[-1])
-            and self.validator == "additionalProperties"
-        ):
-            altair_class = schema_path.split(".")[-1]
-            vegalite_core_class = getattr(vegalite, schema_path.split(".")[-1])
-            param_dict_keys = inspect.signature(vegalite_core_class).parameters.keys()
+        if self.validator == "additionalProperties":
+            param_dict_keys = inspect.signature(cls).parameters.keys()
             param_names_table = self._format_params_as_table(param_dict_keys)
 
             # `cleandoc` removes multiline string indentation in the output
@@ -259,18 +254,18 @@ def __str__(self):
                 {}
                 See the help for `{}` to read the full description of these parameters
                 """.format(
-                    altair_class,
+                    cls.__name__,
                     self.message.split("('")[-1].split("'")[0],
                     param_names_table,
-                    altair_class,
+                    cls.__name__,
                 )
             )
         # Use the default error message for all other cases than unknown parameter errors
         else:
             message = self.message
             # Add a summary line when parameters are passed an invalid value
             # For example: "'asdf' is an invalid value for `stack`
-            if hasattr(vegalite, schema_path.split(".")[-1]) and self.absolute_path:
+            if self.absolute_path:
                 # The indentation here must match that of `cleandoc` below
                 message = f"""'{self.instance}' is an invalid value for `{self.absolute_path[-1]}`: