Remove name generation logic from CgVariableConstructor

[EgorkaKulikov]

Description

Name generation in codegen is the responsibility of NameGenerator.
This logic should not be implemented in CgVariableConstructor. API of this class reduced.

How to test

Automated tests

Running utbot-sample pipeline is enough for regression checks.
No new logic in PR.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.