Remove connection costs output

[martacki]

Changes proposed in this Pull Request

Suggest to remove the snakemake output for connection costs, because it creates bugs if a selection of countries is taken that does not contain links, and remove_stubs is set to false.

Also, UA is not part of the synthetic load data, so removing UA there. I noticed the connection_costs bug when running UA only which is why I'm combining those two bugfixes here.

workflow tested also with config.default (all countries) for power and all other sectors.

I'm not aware of any need of the output in other rules.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[koen-vg]

I guess this actually fixes #843 too?

[fneum]

Good to go! This output is never used.

[martacki]

I guess this actually fixes #843 too?

awesome!