Cache book_titleby only for 10 seconds

This should be enough caching when renderering pages like the feed, but
not so much that editing a book or author will not show the updated data.
At least without having to do some clever cache busting.

bookwyrm/models/author.py

@@ -1,8 +1,6 @@
 """ database schema for info about authors """
 import re
 from django.contrib.postgres.indexes import GinIndex
-from django.core.cache import cache
-from django.core.cache.utils import make_template_fragment_key
 from django.db import models
 
 from bookwyrm import activitypub
@@ -37,16 +35,7 @@ class Author(BookDataModel):
     bio = fields.HtmlField(null=True, blank=True)
 
     def save(self, *args, **kwargs):
-        """clear related template caches"""
-        # clear template caches
-        if self.id:
-            cache_keys = [
-                make_template_fragment_key("titleby", [book])
-                for book in self.book_set.values_list("id", flat=True)
-            ]
-            cache.delete_many(cache_keys)
-
-        # normalize isni format
+        """normalize isni format"""
         if self.isni:
             self.isni = re.sub(r"\s", "", self.isni)