Remove unnecessary delays before closing sessions and clients in tests

[dmitrii-ubskii]

What is the goal of this PR?

We clean up the delays that were introduced as a workaround to a segmentation fault in a much older version of Grakn. Current versions of TypeDB don't exhibit the faulty behaviour.

What are the changes implemented in this PR?

The delays were prompted by https://github.com/vaticle/typedb/issues/6135, which appears to have been resolved.

[typedb-bot]

PR Review Checklist

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Trivial Change

• [x] This change is trivial and does not require a code or architecture review.

Code

• [ ] Packages, classes, and methods have a single domain of responsibility.
• [ ] Packages, classes, and methods are grouped into cohesive and consistent domain model.
• [ ] The code is canonical and the minimum required to achieve the goal.
• [ ] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• [ ] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• [ ] The code is algorithmically efficient and scalable for the whole application.

Architecture

• [ ] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• [ ] Any required build and release automations are updated and/or implemented.
• [ ] Any new components follows a consistent style with respect to the pre-existing codebase.
• [ ] The architecture intuitively reflects the application domain, and is easy to understand.
• [ ] The architecture has a well-defined hierarchy of encapsulated components.
• [ ] The architecture is extensible and scalable.