Remove outdated default semantic model references

[Copilot]

Microsoft Fabric lakehouses no longer automatically create a default semantic model. Documentation still references this deprecated behavior.

Changes

• Instructions/Labs/01-lakehouse.md: Removed claim that managed tables "are included in a default semantic model to support data visualizations"
• Instructions/Labs/03b-medallion-lakehouse.md: Removed note stating "you can't use the default semantic model that is automatically created when you create a lakehouse"

Both edits remove references to functionality that no longer exists in the platform.

Original prompt

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This section details on the original issue you should resolve

<issue_title>New lakehouse doesn't contain a default semantic model anymore</issue_title>
<issue_description># Module: Get started with lakehouses in Microsoft Fabric

Lab/Demo: Create a Microsoft Fabric Lakehouse

Task: Clean up resources

At the end of the lab, in the concluding remarks, it says: The managed tables can be queried using SQL, and are included in a default semantic model to support data visualizations.

There is no default semantic model anymore.

</issue_description>

<agent_instructions>Remove the reference to the default semantic model.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes New lakehouse doesn't contain a default semantic model anymore #336

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