Remove METADATA and cleanup galaxy.yml (ansible-collections#70)

* Remove ANSIBLE_METADATA entirely, see ansible/ansible/pull/69454.
Remove `license` field from galaxy.yml, in favor of `license_file`.

This commit was initially merged in https://github.com/ansible-collections/community.aws
See: ansible-collections/community.aws@05672a6

plugins/modules/ec2_vpc_nat_gateway.py

@@ -6,11 +6,6 @@
 __metaclass__ = type
 
 
-ANSIBLE_METADATA = {'metadata_version': '1.1',
-                    'status': ['preview'],
-                    'supported_by': 'community'}
-
-
 DOCUMENTATION = '''
 ---
 module: ec2_vpc_nat_gateway