Remove unused IsolatedExecutionState#unique_id

This was [replaced][1] with `ExecutionWrapper#active_key` and is no longer
used.

[1]: 10c64a4

Author: skipkayhil

activesupport/lib/active_support/isolated_execution_state.rb

@@ -28,10 +28,6 @@ def isolation_level=(level)
         @isolation_level = level
       end
 
-      def unique_id
-        self[:__id__] ||= Object.new
-      end
-
       def [](key)
         if state = @scope.current.active_support_execution_state
           state[key]