Drop unused table backup_blobs in migration

Today, we solved an issue with holger's phone that he couldn't export
his account anymore because during the `VACUUM` the Android system
killed the app because of OOM. The solution was to drop the table
`backup_blobs`, so let's automatically do this in migration

This table was used back in the olden days when we backuped by exporting
the dbfile and then putting all blobs into it. During import, the
`backup_blobs` table should have been dropped, seems like this didn't
work here.

src/sql/migrations.rs

@@ -608,6 +608,11 @@ CREATE INDEX smtp_messageid ON imap(rfc724_mid);
         )
         .await?;
     }
+    if dbversion < 88 {
+        info!(context, "[migration] v87");
+        sql.execute_migration("DROP TABLE IF EXISTS backup_blobs;", 88)
+            .await?;
+    }
 
     Ok((
         recalc_fingerprints,