Drop on_hold_duration column (#12964)

Resolves #10896

### Description
Removes the `on_hold_duration` in the task table

### Acceptance Criteria
- [ ] Code compiles correctly

### Testing Plan
1. Migration runs smoothly
```bash
bin/rails db:migrate
```
2. Tasks no longer have the `on_hold_duration` field
```ruby
Task.last.on_hold_duration
NoMethodError: undefined method `on_hold_duration' for #<JudgeDecisionReviewTask:0x00007fc33b35f6c8>
```

### Database Changes
* [x] Query profiling performed (eyeball Rails log, check bullet and fasterer output)
* [x] #appeals-schema notified with summary and link to this PR

db/migrate/20191213173125_remove_on_hold_duration_from_tasks.rb

@@ -0,0 +1,5 @@
+class RemoveOnHoldDurationFromTasks < ActiveRecord::Migration[5.1]
+  def change
+    safety_assured { remove_column :tasks, :on_hold_duration, :int }
+  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: 20191206024319) do
+ActiveRecord::Schema.define(version: 20191213173125) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1185,7 +1185,6 @@
     t.datetime "closed_at"
     t.datetime "created_at", null: false
     t.text "instructions", default: [], array: true
-    t.integer "on_hold_duration"
     t.integer "parent_id"
     t.datetime "placed_on_hold_at"
     t.datetime "started_at"