[Task]: Clean up DbCache code

### What needs to happen?

The database cache code was originally saving itself to a file called `db.cache`. However, the code to save itself is never called again. It's best to clean up this code as it is never used anyway. There's also no point in looking for the file when instantiating the cache and possibly throwing an error. So a cleanup is in order.

See also: https://github.com/apache/hop/issues/6042

### Issue Priority

Priority: 3

### Issue Component

Component: API

Start by locating the DbCache implementation and inspect the code that reads or writes db.cache during cache instantiation. Review the related issue 6042 for context, then verify that the unused file handling is removed without leaving callers broken.

Written by the indexing model from the issue text.