[CT-3540] Clean up rpc related code

[emmyoop]

Housekeeping

• 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.