[refactor] Remove HTTP/1.1 (gRPC-fallback) support. (#114)

We do not yet have a credible testing strategy for gRPC fallback support
in generated clients. We will reintroduce this feature once we are able
to test it.

gapic/templates/$namespace/$name_$version/$sub/services/$service/transports/__init__.py.j2

@@ -2,28 +2,18 @@
 
 {% block content %}
 import collections
-import typing
 
 from .base import {{ service.name }}Transport
-from .http import {{ service.name }}HttpTransport
+from .grpc import {{ service.name }}GrpcTransport
+
 
 # Compile a registry of transports.
 _transport_registry = collections.OrderedDict()
-
-# gRPC is not guaranteed to be available, because `grpcio` may or may not
-# be installed. If it is available, however, it should be the "first in"
-# (default).
-try:
-    from .grpc import {{ service.name }}GrpcTransport
-    _transport_registry['grpc'] = {{ service.name }}GrpcTransport
-except ImportError:
-    pass
-
-# Always provide an HTTP/1.1 transport.
-_transport_registry['http'] = {{ service.name }}HttpTransport
+_transport_registry['grpc'] = {{ service.name }}GrpcTransport
 
 
 __all__ = (
     '{{ service.name }}Transport',
+    '{{ service.name }}GrpcTransport',
 )
 {% endblock %}