obsolete: drop the legacy `_enabled` variable

Evolve 8.5.0 stopped setting this, and it would have been easier to figure out
why TortoiseHg stopped allowing amends if it would have crashed on the missing
variable.

mercurial/obsolete.py

@@ -93,10 +93,6 b" parsers = policy.importmod(r'parsers')"
 _calcsize = struct.calcsize
 propertycache = util.propertycache
 
-# the obsolete feature is not mature enough to be enabled by default.
-# you have to rely on third party extension extension to enable this.
-_enabled = False
-
 # Options for obsolescence
 createmarkersopt = 'createmarkers'
 allowunstableopt = 'allowunstable'
@@ -124,11 +120,6 b' def _getoptionvalue(repo, option):'
         if 'all' in result:
             return True
 
-        # For migration purposes, temporarily return true if the config hasn't
-        # been set but _enabled is true.
-        if len(result) == 0 and _enabled:
-            return True
-
         # Temporary hack for next check
         newconfig = repo.ui.config('experimental', 'evolution.createmarkers')
         if newconfig: