Scripts: Fix linting issues in [octez-init-sandbox-client.sh]

What

A first merge request to refactor the octez-init-sandbox-client.sh.

This one fixes the linting issues in this scripts.

A follop merge request will refactor the script to handle protocol related binaires better.

Why

Shellcheck fails on src/bin_node/octez-init-sandbox-client.sh

How

• Removes unused functions
• Removes unuses variables
• Fix minor linting issues

Manually testing the MR

Run shellcheck on the src/bin_client/octez-init-sandbox-client.sh

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR