Removed references to build_from_flow and block-based deployments for Prefect 3.0

[mthatt]

Removed mentions of build_from_flow everywhere except in ''Upgrade from Agents to Workers" guide. Removed 'Block and Agent-Based deployments" section.

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[billpalombi]

Nice!

[github-actions]

This pull request is stale because it has been open 14 days with no activity. To keep this pull request open remove stale label or comment.

[billpalombi]

@mthatt, we can close this out. These changes are now replaced in the 3.0 docs.