Remove thumbnail generation task

Now that the thumbnail generation task has been ported to a worker and that we dropped the code that calls the task, we can finish the job.

tests/samples/build_entities/create_blocks.json

-[
-    {
-        "xml_file": "create_blocks.xml",
-        "corpus_id": "corpus_id",
-        "page_id": "page_id",
-        "transcriptions": [
-            {
-                "transcription_id": "transcription_0",
-                "index_region": 0
-            },
-            {
-                "transcription_id": "transcription_1",
-                "index_region": 1
-            }
-        ]
-    }
-]
\ No newline at end of file