[21086] Remove `string_convert` header and source

[MiguelCompany]

Description

This PR removes string_convert.hpp and string_convert.cpp, which are not used in the project.

Contributor Checklist

• [x] Commit messages follow the project guidelines.
• N/A 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)
• NO Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• NO 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

• [x] The PR has a milestone assigned.
• [x] The title and description correctly express the PR's purpose.
• [x] Check contributor checklist is correct.
• [x] Check CI results: changes do not issue any warning.
• [x] Check CI results: failing tests are unrelated with the changes.

[MiguelCompany]

@richiprosima please check style

[MiguelCompany]

@richiprosima Please test_3 discovery-server