Make the deleting of the blob consistent

Deleting blobs linked to sample types is done in the same manner as the
blobs linked to other assets.

lib/seek/acts_as_asset/content_blobs.rb

@@ -5,7 +5,7 @@ module ContentBlobs
       module ClassMethods
         extend ActiveSupport::Concern
         included do
-          after_destroy :mark_deleted_content_blobs, unless: -> { self.class == SampleType }
+          after_destroy :mark_deleted_content_blobs
         end
       end
 

lib/seek/templates/sample_type_template_concerns.rb

@@ -7,7 +7,7 @@ module SampleTypeTemplateConcerns
       included do
         after_save :queue_template_generation, :queue_sample_type_update_job
         validate :validate_template_file
-        has_one :content_blob, as: :asset, dependent: :destroy
+        has_one :content_blob, as: :asset
         alias_method :template, :content_blob
         has_task :template_generation
         extend ClassMethods

test/unit/sample_type_test.rb

@@ -454,13 +454,13 @@ def setup
         sample_type.save!
         blob = sample_type.content_blob
 
-        assert_difference('ContentBlob.count', -1) do
-          assert_difference('SampleType.count', -1) do
-            sample_type.destroy
-          end
+        refute blob.deleted?
+
+        assert_difference('SampleType.count', -1) do
+          sample_type.destroy
         end
 
-        assert blob.destroyed?
+        assert blob.deleted?
       end
     end
   end