Drop look-before-leap use of protobuf-internal type checker.

The protobuf itself will raise the error when an out-of-bounds int/long
value is assigned.

Addresses:
protocolbuffers/protobuf#1307 (comment).

gcloud/datastore/helpers.py

@@ -19,7 +19,6 @@
 
 import datetime
 
-from google.protobuf.internal.type_checkers import Int64ValueChecker
 import six
 
 from gcloud._helpers import _datetime_from_microseconds
@@ -30,8 +29,6 @@
 
 __all__ = ('entity_from_protobuf', 'key_from_protobuf')
 
-INT_VALUE_CHECKER = Int64ValueChecker()
-
 
 def find_true_project(project, connection):
     """Find the true (unaliased) project.
@@ -318,8 +315,7 @@ def _pb_attr_value(val):
     elif isinstance(val, float):
         name, value = 'double', val
     elif isinstance(val, six.integer_types):
-        INT_VALUE_CHECKER.CheckValue(val)   # Raise an exception if invalid.
-        name, value = 'integer', int(val)  # Always cast to an integer.
+        name, value = 'integer', val
     elif isinstance(val, six.text_type):
         name, value = 'string', val
     elif isinstance(val, (bytes, str)):

gcloud/datastore/test_helpers.py

@@ -457,14 +457,6 @@ def test_long(self):
         self.assertEqual(name, 'integer_value')
         self.assertEqual(value, must_be_long)
 
-    def test_long_too_small(self):
-        too_small = -(1 << 63) - 1
-        self.assertRaises(ValueError, self._callFUT, too_small)
-
-    def test_long_too_large(self):
-        too_large = 1 << 63
-        self.assertRaises(ValueError, self._callFUT, too_large)
-
     def test_native_str(self):
         import six
         name, value = self._callFUT('str')