planemo shed_upload --tar_only doesn't need --shed_target

e.g.

```
$ planemo --version
planemo, version 0.20.1.dev0
$ planemo shed_upload --tar_only ~/repositories/pico_galaxy/tools/coverage_stats/
Usage: planemo shed_upload [OPTIONS] PROJECT

Error: default_shed_target set to None, must specify a value for --shed_target to run this command.
```

Since the construction of the tar-ball is offline, there should be no reason to require a target Tool Shed (although in general this is needed for the `planemo shed_upload` command).

Workaround, give a dummy `--shed_target` or `-t` option:

```
$ planemo shed_upload -t xxx --tar_only ~/repositories/pico_galaxy/tools/coverage_stats/
cp '/tmp/tmpYuuev5' 'shed_upload.tar.gz'
```