Revert "Don't return hidden templates in API service template responses"

Filtering out hidden templates requires all existing templates to
have `hidden` flag set, which can only be done by a migration after
the code that sets the flag to `False` by default for new templates
has been released.

This removes the filtering logic until the migration has been released.

tests/app/conftest.py

@@ -216,7 +216,6 @@ def sample_template(
     template_type="sms",
     content="This is a template:\nwith a newline",
     archived=False,
-    hidden=False,
     subject_line='Subject',
     user=None,
     service=None,
@@ -238,7 +237,6 @@ def sample_template(
         'service': service,
         'created_by': created_by,
         'archived': archived,
-        'hidden': hidden,
         'process_type': process_type
     }
     if template_type in ['email', 'letter']: