Port: Not dropping table for incremental full refresh with delta (#124)

### Description

Ports the upstream change: Not dropping table for incremental full refresh with delta (dbt-labs/dbt-spark#287)

CHANGELOG.md

@@ -1,5 +1,11 @@
 ## dbt-databricks 1.2.0 (Release TBD)
 
+### Fixes
+- Incremental materialization updated to not drop table first if full refresh for delta lake format, as it already runs _create or replace table_ ([#286](https://github.com/dbt-labs/dbt-spark/issues/286), [#287](https://github.com/dbt-labs/dbt-spark/pull/287/))
+
+### Contributors
+- [@grindheim](https://github.com/grindheim) ([#287](https://github.com/dbt-labs/dbt-spark/pull/287/))
+
 ## dbt-databricks 1.1.1 (Release TBD)
 
 ### Features

dbt/include/databricks/macros/materializations/incremental/incremental.sql

@@ -26,10 +26,14 @@
 
   {{ run_hooks(pre_hooks) }}
 
+  {% set is_delta = (file_format == 'delta' and existing_relation.is_delta) %}
+
   {% if existing_relation is none %}
     {% set build_sql = create_table_as(False, target_relation, sql) %}
   {% elif existing_relation.is_view or full_refresh_mode %}
-    {% do adapter.drop_relation(existing_relation) %}
+    {% if not is_delta %} {#-- If Delta, we will `create or replace` below, so no need to drop --#}
+      {% do adapter.drop_relation(existing_relation) %}
+    {% endif %}
     {% set build_sql = create_table_as(False, target_relation, sql) %}
   {% else %}
     {% do run_query(create_table_as(True, tmp_relation, sql)) %}