Remove redundant functions for Documents

There's no need for a `string` function since one already
exists for Nodes.

Now the root node is always stored on the document. This
means callers of `ts_document_root_node` don't need to release
its return value.

spec/runtime/languages/language_specs.cc

@@ -28,7 +28,7 @@ describe("Languages", [&]() {
       for (auto &entry : test_entries_for_language(language_name)) {
         it(entry.description.c_str(), [&]() {
           ts_document_set_input_string(doc, entry.input.c_str());
-          auto doc_string = ts_document_string(doc);
+          auto doc_string = ts_node_string(ts_document_root_node(doc));
           AssertThat(doc_string, Equals(entry.tree_string.c_str()));
           free((void *)doc_string);
         });