Remove schema override for variable default value

[shreyas-goenka]

Changes

This PR:

1. Removes the custom added in https://github.com/databricks/cli/pull/1396/files for variables.*.default. It's no longer needed because with complex variables (https://github.com/databricks/cli/pull/1467) default has a type of any.
2. Retains, and extends the override on targets.*.variables.*. Target override values can now be complex objects, not just primitive values.

Tests

Manually

Before: Only primitive types were allowed.

After: An empty JSON schema is generated. All YAML values are acceptable.

[shreyas-goenka]

@pietern My bad, overlooked that. Fixed it now.

target overrides:

target override schema:

          "variables": {
            "type": "object",
            "additionalProperties": {}
          },`