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/flags.rb

@@ -1,19 +1,19 @@
 section "Flagging Debates & Comments" do
   40.times do
-    debate = Debate.all.sample
-    flagger = User.where.not(id: debate.author_id).all.sample
+    debate = Debate.sample
+    flagger = User.where.not(id: debate.author_id).sample
     Flag.flag(flagger, debate)
   end
 
   40.times do
-    comment = Comment.all.sample
-    flagger = User.where.not(id: comment.user_id).all.sample
+    comment = Comment.sample
+    flagger = User.where.not(id: comment.user_id).sample
     Flag.flag(flagger, comment)
   end
 
   40.times do
-    proposal = Proposal.all.sample
-    flagger = User.where.not(id: proposal.author_id).all.sample
+    proposal = Proposal.sample
+    flagger = User.where.not(id: proposal.author_id).sample
     Flag.flag(flagger, proposal)
   end
 end