Refactor notifiable_available? method

This method was calling `check_availability`, which returned a boolean
value and caused a `Naming/PredicateMethod` when upgrading rubocop.

So we're changing the logic a little bit to remove the
`check_availability` method and merge the tests of `check_availability`
and `notifiable_available?` (which were almost identical) together.

app/models/notification.rb

@@ -10,8 +10,7 @@ class Notification < ApplicationRecord
   scope :recent,      -> { order(id: :desc) }
   scope :for_render,  -> { includes(:notifiable) }
 
-  delegate :notifiable_title, :notifiable_body, :notifiable_available?,
-           :check_availability, :linkable_resource,
+  delegate :notifiable_title, :notifiable_body, :notifiable_available?, :linkable_resource,
            to: :notifiable, allow_nil: true
 
   def mark_as_read