Removal of code related to the Split Velocity Method

[ArneVoss]

Proposed Changes

To simulate a gust in CFD, typically the Field Velocity Method is used. This method prescribes a given velocity on each node and neglects any influence of the aircraft on the gust. The Split Velocity Method is an approach that allows to capture the influence of the aircraft on the gust.

As far a I understand, the Split Velocity Method is only partially implemented in SU2 and isn't used for any gust type because all gust derivatives are zero / commented out in the code. This is in line with Padron et al. (https://doi.org/10.2514/6.2014-3013) who report that "... combining the GCL with the SVM does not work."

This pull request cleans up / removes the unused code.

Related Work

The issue was identified during the work on pull request #1954

PR Checklist

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• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.

[ArneVoss]

Thank you for your corrections, Pedro, that's a quick pull request :)