revlog: remove legacy usage of `_compengine`

All core code is now getting the setting from the FeatureConfig object.

mercurial/revlog.py

@@ -976,7 +976,7 b' class revlog:'
 
     @util.propertycache
     def _compressor(self):
-        engine = util.compengines[self._compengine]
+        engine = util.compengines[self.feature_config.compression_engine]
         return engine.revlogcompressor(self._compengineopts)
 
     @util.propertycache

mercurial/revlogutils/docket.py

@@ -330,7 +330,9 b' def default_docket(revlog, version_heade'
     rl_version = version_header & 0xFFFF
     if rl_version not in (constants.REVLOGV2, constants.CHANGELOGV2):
         return None
-    comp = util.compengines[revlog._compengine].revlogheader()
+    comp = util.compengines[
+        revlog.feature_config.compression_engine
+    ].revlogheader()
     docket = RevlogDocket(
         revlog,
         version_header=version_header,