[24015] Remove unused keep_alive logic

[cferreiragonz]

Description

This PR removes unused code regarding an unimplemented keep alive feature.

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• N/A: Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A: Any new/modified methods have been properly documented using Doxygen.
• N/A: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
• N/A: Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• N/A: Changes are API compatible.
• N/A: New feature has been added to the versions.md file (if applicable).
• N/A: New feature has been documented/Current behavior is correctly described in the documentation.
• N/A: Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.

[emiliocuestaf]

LGTM with green CI.

[mergify]

🧪 CI Insights

Here's what we observed from your CI run for 81946cc.

❌ Job Failures

Pipeline	Job	Health on master	Retries	🔍 CI Insights	📄 Logs
Fast DDS MacOS CI	mac-ci / fastdds_test ()		0	View	View
Fast DDS Ubuntu CI	ubuntu-ci / fastdds_test (RelWithDebInfo)		1	View	View
Fast DDS Windows CI	windows-ci / fastdds_test (RelWithDebInfo, examples), v143		0	View	View