Remove the unused field from the database.

In the PR #1140 this field was added in commit ad697cd2c1, but in this same
PR in commit 28d12fe55 all the related functionality that had been added was
removed but the field was not removed.

db/migrate/20230523090028_remove_email_on_proposal_notification_from_users.rb

@@ -0,0 +1,5 @@
+class RemoveEmailOnProposalNotificationFromUsers < ActiveRecord::Migration[6.0]
+  def change
+    remove_column :users, :email_on_proposal_notification, :boolean, default: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2023_02_06_141152) do
+ActiveRecord::Schema.define(version: 2023_05_23_090028) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -1630,7 +1630,6 @@ ActiveRecord::Schema.define(version: 2023_02_06_141152) do
     t.string "redeemable_code"
     t.string "gender", limit: 10
     t.datetime "date_of_birth"
-    t.boolean "email_on_proposal_notification", default: true
     t.boolean "email_digest", default: true
     t.boolean "email_on_direct_message", default: true
     t.boolean "official_position_badge", default: false