Remove the non-destructive flag and all associated provisions

[drewbanin]

Feature

Feature description

Let's delete some code!

The --non-destructive flag was originally intended to work around Redshift's bound views. With the advent of late binding views, this is no longer necessary. Moreover, --non-destructive never worked in an ideal way.

Some problems:

• it just skips views, making it difficult to keep the database in sync with code changes
• it uses a truncate for tables which commits the open transaction. This can lead to query inconsistency from external queriers.
• it makes materialization code super complicated on pretty much every database dbt supports

Caveats

We'll need to:

• remove the --non-destructive flag
• cleanse materializations of non-destructive logic (might be tricky, but will be a huge improvement)
• note specific places where we've needed to make weird workarounds for non-destructive builds, like here

This is one of the oldest and most neglected features in dbt, so we should take extra care to see if there are any lingering code paths that become irrelevant once non-destructive mode doesn't exist anymore.

Who will this benefit?

dbt maintainers, folks that want to implement custom materializations