curthooks running apt-config unconditionnally

[Olivier Gayot]

The conditional execution of apt_config.handle_apt from curthooks builtins has been broken for a while. This results in parts of the apt configuration (e.g., proxy settings, apt preferences...) being discarded mid install when using Subiquity.

A regression was introduced in 2016 when adding a mechanism to translate things like:

```
                                  apt:
    debconf_selections: => debconf_selections:
      foobar foobar
```

The function responsible for calling handle_apt is as follows:

def do_apt_config(cfg, target):
    cfg = apt_config.translate_old_apt_features(cfg)
    apt_cfg = cfg.get("apt")
    if apt_cfg is not None:
        LOG.info("curthooks handling apt to target %s with config %s",
                 target, apt_cfg)
        apt_config.handle_apt(apt_cfg, target)
    else:
        LOG.info("No apt config provided, skipping")

Sadly, the implementation of translate_old_apt_features made is so that the `if apt_cfg is not None:` condition is always true.

This bug is the root cause:
https://bugs.launchpad.net/subiquity/+bug/1970409
https://code.launchpad.net/~ogayot/curtin/+git/curtin/+merge/414408

I suggest we revert the changes after merging the relevant fixes