Remove create_snowplow_manifest_schema step

[bill-warner]

Currently

• The snowplow_web_base_sessions_lifecycle_manifest model's schema is defined by the snowplow__manifest_custom_schema variable. Since this model is a 'proper' dbt model, dbt will create this schema before doing anything else in the run if it doesn't exist already.
• The snowplow_web_incremental_manifest table's schema is also defined by the snowplow__manifest_custom_schema variable. This however is a manually created table via DDL.
• Since the manifest schema is created by dbt, when we go to create the snowplow_web_incremental_manifest table, the manifest schema exists.

Potential implementation

• A user overrides the snowplow_web_base_sessions_lifecycle_manifest model's schema but not by changing the snowplow__manifest_custom_schema variable accordingly (this can be achieved by amending the dbt_project.yml)
• The snowplow_web_incremental_manifest tables schema is still defined by the snowplow__manifest_custom_schema variable. This schema will no longer be generated automatically by dbt since the snowplow_web_base_sessions_lifecycle_manifest model is not using the same schema. This would cause the CREATE TABLE DDL to fail as the schema doesn't exist.
• This is why the create_snowplow_manifest_schema step is included as a fail safe.

Issue

• The adapter.create_schema method I use here to manually create the schema, under the hood calls CREATE SCHEMA IF NOT EXISTS.
• If the user does not have schema creation permissions this will fail.
• Despite dbt recommending having such permissions for all dbt users, one of our customers using the package does not allow their dbt users to have such permissions. Rather a super user manually creates all the schemas they need for dbt.
• This meant every time they ran the package it failed due to this step, despite the required schema already existing.

Fix

• We should remove the schema creation step to be sure the package will run irrespective of the users permissions.