[alxbilger]

In the known implementations of BaseMechanicalState::endIntegration, we clear the vector with id externalForce. Instead of having a specific method endIntegration, this PR suggests to simply call resetVec instead. If we do that, endIntegration becomes dead and can be deprecated.

Related to #6362

[with-all-tests]

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).