fix: consolidate dependencies in setup.py (#422)

Author: busunkim96

gapic/templates/setup.py.j2

@@ -16,10 +16,7 @@ setuptools.setup(
     platforms='Posix; MacOS X; Windows',
     include_package_data=True,
     install_requires=(
-        'google-auth >= 1.14.0',
-        'google-api-core >= 1.17.0, < 2.0.0dev',
-        'googleapis-common-protos >= 1.5.8',
-        'grpcio >= 1.10.0',
+        'google-api-core[grpc] >= 1.17.0, < 2.0.0dev',
         'proto-plus >= 0.4.0',
     {%- if api.requires_package(('google', 'iam', 'v1')) %}
         'grpc-google-iam-v1',