Remove manual gap root overwrite (gap -l <paths>)

[tobiasdiez]

Currently, the gap root path is used in two places:

• when invoking gap, using -l <paths>
• for the gap workspace feature

With the changes in this PR, only the second usage remains. This will make it impossible to use sage's gap_packages with the system gap, but it simplifies a lot of things.

For more discussion see tobiasdiez#19.

📝 Checklist

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⌛ Dependencies