Remove the `--packaging-format` option for Linux System packaging

### Describe the bug

While it may be possible to build packages that are not native to the distro, doing this in practice does not work.

Even if you build a deb in Fedora, you can't install it in to another Fedora environment; none of the dependent packages will be available.

Building a pkg with `makepkg` requires all the dependencies to already be installed...but how can I install them if I'm not on Arch?

### Steps to reproduce

Attempt to build a package that is not native to the target.

### Expected behavior

The packaging format is chosen based on the target distro....which is the default (and sane) behavior anyway.

### Screenshots

_No response_

### Environment

- Operating System: fedora 37
- Python version: 3.11.4
- Software versions:
- Briefcase: `0.3.16.dev58+g5483fd9a`

### Logs

_No response_

### Additional context

_No response_

Start by locating the `--packaging-format` option and the Linux system packaging entry points. Check how the option currently affects distro-specific packaging, then verify that removing it preserves selection based on the target distribution and that non-native formats can no longer be requested.

Written by the indexing model from the issue text.