Don't use s3 bucket_head to check for bucket existence

This is about
piskvorky#314

This code caused problems when you have write permission to an s3
bucket path but not read permission to the bucket root.

This code was added to prevent implicit bucket creation (to fix issue
154) but it appears that it is no longer needed.

smart_open/s3.py

@@ -418,14 +418,6 @@ def __init__(
             multipart_upload_kwargs = {}
 
         s3 = session.resource('s3', **resource_kwargs)
-
-        #
-        # https://stackoverflow.com/questions/26871884/how-can-i-easily-determine-if-a-boto-3-s3-bucket-resource-exists
-        #
-        try:
-            s3.meta.client.head_bucket(Bucket=bucket)
-        except botocore.client.ClientError:
-            raise ValueError('the bucket %r does not exist, or is forbidden for access' % bucket)
         self._object = s3.Object(bucket, key)
         self._min_part_size = min_part_size
         self._mp = self._object.initiate_multipart_upload(**multipart_upload_kwargs)