Remove redundant .all in .all.sample calls

Rubocop doesn't detect this method because we've added it in the
ApplicationRecord class, but we're removing the `.all` calls for
consistency.

db/dev_seeds/comments.rb

@@ -1,8 +1,8 @@
 section "Commenting Investments, Debates & Proposals" do
   %w[Budget::Investment Debate Proposal].each do |commentable_class|
     100.times do
-      commentable = commentable_class.constantize.all.sample
-      Comment.create!(user: User.all.sample,
+      commentable = commentable_class.constantize.sample
+      Comment.create!(user: User.sample,
                       created_at: rand(commentable.created_at..Time.current),
                       commentable: commentable,
                       body: Faker::Lorem.sentence)
@@ -12,8 +12,8 @@ end
 
 section "Commenting Comments" do
   200.times do
-    parent = Comment.all.sample
-    Comment.create!(user: User.all.sample,
+    parent = Comment.sample
+    Comment.create!(user: User.sample,
                     created_at: rand(parent.created_at..Time.current),
                     commentable_id: parent.commentable_id,
                     commentable_type: parent.commentable_type,