fix(gapic-generator): use flat_ref_types in unit test templates and delete remove-unused-imports post-processor

Author: ohmayr

.librarian/generator-input/client-post-processing/remove-unused-imports.yaml

@@ -57,7 +57,7 @@ from google.longrunning import operations_pb2 # type: ignore
 {% endif %}
 from google.api_core import gapic_v1
 {% for method in service.methods.values() %}
-{% for ref_type in method.ref_types
+{% for ref_type in method.flat_ref_types
    if not ((ref_type.ident.python_import.package == ('google', 'api_core') and ref_type.ident.python_import.module == 'operation')
            or ref_type.ident.python_import.package == ('google', 'protobuf') and ref_type.ident.python_import.module == 'empty_pb2') %}
 {{ ref_type.ident.python_import }}

packages/gapic-generator/gapic/ads-templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -86,7 +86,7 @@ from google.longrunning import operations_pb2 # type: ignore
 {% endif %}
 from google.api_core import gapic_v1
 {% for method in service.methods.values() %}
-{% for ref_type in method.ref_types
+{% for ref_type in method.flat_ref_types
    if not (ref_type.ident.python_import.package == ('google', 'api_core') and ref_type.ident.python_import.module == 'operation') %}
 {{ ref_type.ident.python_import }}
 {% endfor %}