[CT-3540] Clean up rpc related code

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

`dbt-rpc` is EOL but there is some code that persists and needs to be cleaned up.

### Acceptance criteria

All code for dbt-rpc has been removed
- rpc node type
- possibly these artifacts
- remove tests directly associated with this code
- all existing test continue to pass

### Suggested Tests

No new tests need to be added as this is just removing dead code. Some existing tests may need to be cleaned up.

### Impact to Other Teams

None

### Will backports be required?

No

### Context

This is dead code that just clutters the codebase. We have several "this is rpc - should it be removed" type comments throughout the code that would be nice to clean up.