The `scylla_tables` function gives a different schema definition
for the `system_schema.scylla_tables` table, depending on whether
certain schema features are enabled or not.
The way it was implemented, we had to write `θ(2^n)` amount
of code and comments to handle `n` features.
Refactor it so that the amount of code we have to write to handle `n`
features is `θ(n)`.
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