config: remove `free` ptr from `git_config_entry`

[ethomson]

While looking at the documentation, it became clear that the git_config_entry function was strange...

This is a leftover leaky abstraction. If consumers aren't meant to call the free function then they shouldn't see the free function. Move it out into a git_config_backend_entry that is, well, produced by the config backends.

This makes our code messier but is an improvement for consumers.