[fix] Remove some unused imports. (#177)

This removes five imports from generated code that are
consistently unused.

Author: Luke Sneeringer

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/services/$service/client.py.j2

@@ -2,7 +2,7 @@
 
 {% block content %}
 from collections import OrderedDict
-from typing import Dict, Mapping, Optional, Sequence, Tuple, Type, Union
+from typing import Dict, Sequence, Tuple, Type, Union
 import pkg_resources
 
 from google.api_core import exceptions        # type: ignore

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/services/$service/pagers.py.j2

@@ -6,7 +6,6 @@
 {# This lives within the loop in order to ensure that this template
    is empty if there are no paged methods.
  -#}
-import copy
 from typing import Any, Callable, Iterable
 
 {% filter sort_lines -%}

packages/gapic-generator/gapic/templates/$namespace/$name_$version/$sub/services/$service/transports/grpc.py.j2

@@ -1,7 +1,7 @@
 {% extends '_base.py.j2' %}
 
 {% block content %}
-from typing import Callable, Dict, Sequence, Tuple
+from typing import Callable, Dict
 
 from google.api_core import grpc_helpers   # type: ignore
 {%- if service.has_lro %}