[AIRFLOW-2266] Remove google-cloud-dataflow dependency

This is caused due to the fact that the latest release (2.4) for apache-beam[gcp] is not available for Python 3.x. Also as we are using Google's discovery based API for all google cloud related commands we don't require to import google-cloud-dataflow package

setup.py

@@ -133,7 +133,6 @@ def write_version(filename=os.path.join(*['airflow',
     'google-api-python-client>=1.5.0, <1.6.0',
     'oauth2client>=2.0.2, <2.1.0',
     'PyOpenSSL',
-    'google-cloud-dataflow>=2.2.0',
     'pandas-gbq'
 ]
 hdfs = ['snakebite>=2.7.8']
@@ -206,8 +205,7 @@ def write_version(filename=os.path.join(*['airflow',
 # Snakebite & Google Cloud Dataflow are not Python 3 compatible :'(
 if PY3:
     devel_ci = [package for package in devel_all if package not in
-                ['snakebite>=2.7.8', 'snakebite[kerberos]>=2.7.8',
-                 'google-cloud-dataflow>=2.2.0']]
+                ['snakebite>=2.7.8', 'snakebite[kerberos]>=2.7.8']]
 else:
     devel_ci = devel_all