Revert "config: enable data index caching by default"

This reverts commit 1af63acae5.

dvc/config_schema.py

@@ -264,8 +264,8 @@ SCHEMA = {
     # section for experimental features
     "feature": {
         Optional("machine", default=False): Bool,
-        # enabled by default. It's of no use, kept for backward compatibility.
         Optional("data_index_cache", default=False): Bool,
+        # enabled by default. It's of no use, kept for backward compatibility.
         Optional("parametrization", default=True): Bool,
     },
     "plots": {

dvc/repo/__init__.py

@@ -381,6 +381,9 @@ class Repo:
     def data_index(self) -> Optional["DataIndex"]:
         from dvc_data.index import DataIndex
 
+        if not self.config["feature"].get("data_index_cache"):
+            return None
+
         if not self.index_db_dir:
             return None