Don't delete _b wildcard field

We need this for some fields. We didn't realise it was missing because
due to SOLR-17834, our overlay to disable auto schema was broken.
Do the minimal upgrade necessary for now (just fix the schema), we'll
upgrade to solr 9.10 when we have the chance to test it

Author: alastair

utils/search/schema/delete_default_fields.json

@@ -44,7 +44,6 @@
       "*_ss",
       "*_l",
       "*_ls",
-      "*_b",
       "*_bs",
       "*_f",
       "*_fs",