Remove unnecessary IntermediateSnapshotNode and EmptySnapshotConfig

[gshank]

resolves #10326

Problem

We no longer need the IntermediateSnapshotNode and EmptySnapshotConfig classes, or the IntermediateNode typevar.

Solution

Remove them.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• [x] This PR includes type annotations for new and modified functions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.78%. Comparing base (71a8a41) to head (7853431).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10332 +/- ## ========================================== - Coverage 88.83% 88.78% -0.06% ========================================== Files 180 180 Lines 22490 22479 -11 ========================================== - Hits 19980 19958 -22 - Misses 2510 2521 +11 ``` | [Flag](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10332/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10332/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `86.08% <100.00%> (-0.11%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10332/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.17% <100.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more.

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