Remove operations

[beckjake]

Feature

We should remove the concept of operations on the jinja level and replace them + adapter.run_operation() with adapter.execute_macro(). The node type Operation should remain.

Feature description

This should be totally safe to remove: dbt currently defines two operations and does not make operations available to users in any way. Both just execute the contents of existing macros, get_catalog and get_relations, which do all the hard work.

There's a bunch of supporting machinery in the parser/loader that can get ripped out as well. Operations will still exist in the graph, as hooks have been operations before this jinja construct existed, and that's fine.

Other issues to keep in mind with respect to this:

• this should not be supplied in an operation context #878/put "this" var back into operation context #879 - This is only slightly relevant
• operations #582 - This is basically unchanged except we'd just let you execute a 'macro' instead of an 'operation'.

Who will this benefit?

This will make dbt a bit more internally consistent and reduce the amount of things users need to understand if they want to implement adapters.