Add and apply DuplicateMethods rubocop rule

These methods were defined with `attr_reader` (or accessor in some
cases) and then they were redefined.

.rubocop_basic.yml

@@ -120,6 +120,9 @@ Layout/TrailingWhitespace:
 Lint/AmbiguousRegexpLiteral:
   Enabled: true
 
+Lint/DuplicateMethods:
+  Enabled: true
+
 Lint/LiteralAsCondition:
   Enabled: true
 

app/models/budget/result.rb

@@ -1,7 +1,7 @@
 class Budget
   class Result
 
-    attr_accessor :budget, :heading, :money_spent, :current_investment
+    attr_accessor :budget, :heading, :current_investment
 
     def initialize(budget, heading)
       @budget = budget

lib/comment_tree.rb

@@ -2,7 +2,7 @@ class CommentTree
 
   ROOT_COMMENTS_PER_PAGE = 10
 
-  attr_reader :root_comments, :commentable, :page, :order
+  attr_reader :commentable, :page, :order
 
   def initialize(commentable, page, order = "confidence_score", valuations: false)
     @commentable = commentable

lib/email_digest.rb

@@ -1,6 +1,6 @@
 class EmailDigest
 
-  attr_accessor :user, :notifications
+  attr_accessor :user
 
   def initialize(user)
     @user = user