Enable Rubocop cops for invalid access modifiers

This enables following cops:

Check for useless access modifiers

  Lint/UselessAccessModifier

Checks for attempts to use `private` or `protected` to set the
visibility of a class method, which does not work.

  Lint/IneffectiveAccessModifier

This also disables two false possitives in concerns.

.rubocop.yml

@@ -506,6 +506,15 @@ Metrics/PerceivedComplexity:
 
 #################### Lint ################################
 
+# Checks for useless access modifiers.
+Lint/UselessAccessModifier:
+  Enabled: true
+
+# Checks for attempts to use `private` or `protected` to set the visibility
+# of a class method, which does not work.
+Lint/IneffectiveAccessModifier:
+  Enabled: false
+
 # Checks for ambiguous operators in the first argument of a method invocation
 # without parentheses.
 Lint/AmbiguousOperator:

.rubocop_todo.yml

@@ -19,10 +19,6 @@ Lint/AssignmentInCondition:
 Lint/HandleExceptions:
   Enabled: false
 
-# Offense count: 21
-Lint/IneffectiveAccessModifier:
-  Enabled: false
-
 # Offense count: 2
 Lint/Loop:
   Enabled: false
@@ -48,10 +44,6 @@ Lint/UnusedBlockArgument:
 Lint/UnusedMethodArgument:
   Enabled: false
 
-# Offense count: 11
-Lint/UselessAccessModifier:
-  Enabled: false
-
 # Offense count: 12
 # Cop supports --auto-correct.
 Performance/PushSplat:

app/models/concerns/token_authenticatable.rb

@@ -20,7 +20,7 @@ module TokenAuthenticatable
       @token_fields || []
     end
 
-    private
+    private # rubocop:disable Lint/UselessAccessModifier
 
     def add_authentication_token_field(token_field)
       @token_fields = [] unless @token_fields

lib/gitlab/ci/config/node/configurable.rb

@@ -36,7 +36,7 @@ module Gitlab
               Hash[(@nodes || {}).map { |key, factory| [key, factory.dup] }]
             end
 
-            private
+            private # rubocop:disable Lint/UselessAccessModifier
 
             def node(symbol, entry_class, metadata)
               factory = Node::Factory.new(entry_class)