Remove unused licenses: python-nvd3 & python-slugify (apache#15860)

This was added when we vendorized nvd3 and slugify: apache@e36bdef

but we forgot to remove it in apache#9136

Author: kaxil

NOTICE

@@ -12,13 +12,3 @@ This product contains a modified portion of 'Hue' developed by Cloudera, Inc.
 (https://github.com/cloudera/hue/).
 
 * Copyright 2009-2017 Cloudera Inc.
-
-python-slugify:
----------------
-
-* Copyright (c) Val Neekman @ Neekware Inc. http://neekware.com
-
-python-nvd3:
-------------
-
-* Copyright (c) 2013 Arezqui Belaid <[email protected]> and other contributors