Revert "Allow polars schema --datatype-list to be used without pipeline inp…"

This reverts commit fb691c0da5.

crates/nu_plugin_polars/src/dataframe/command/core/schema.rs

@@ -25,7 +25,7 @@ impl PluginCommand for SchemaCmd {
     fn signature(&self) -> Signature {
         Signature::build(self.name())
             .switch("datatype-list", "creates a lazy dataframe", Some('l'))
-            .input_output_type(Type::Any, Type::record())
+            .input_output_type(Type::Custom("dataframe".into()), Type::record())
             .category(Category::Custom("dataframe".into()))
     }