Add a quick fix for underwater_fraction calculations

[ekatef]

Closes #765

Remove re-calculation of underwater_fraction after augmentation to fix problems which can arise for networks with HVDC lines due to empty n.links.geometry after simplification.

Augmentation of networks with HVDC part should be anyway improved which may be tracked in #765.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[ekatef]

@davide-f that is "raw fix" implementation we discussed in #768.
Ready for review :)

[davide-f]

Perfect :) no need for release note I'd say.
Merging