fix: remove duplication in simple extensions schema

[mbrobbel]

While working on generating deserialization code based on the simple extensions
schema in substrait-io/substrait-rs#27 I got a failing
test on extension_types.yaml.
It seems that the generated deserialization code tripped over the schema for
structure in items in the types array.

Maybe I'm misunderstanding something about the schema here, but I think we can
directly use $ref: "#/$defs/type" for structure here.

According to
https://json-schema.org/understanding-json-schema/reference/object.html#id5:

The additionalProperties keyword is used to control the handling of extra
stuff, that is, properties whose names are not listed in the properties
keyword or match any of the regular expressions in the patternProperties
keyword. By default any additional properties are allowed.

The value of the additionalProperties keyword is a schema that will be used
to validate any properties in the instance that are not matched by
properties or patternProperties. Setting the additionalProperties schema
to false means no additional properties will be allowed.

[github-actions]

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