drop session_key compression

[jrief]

Until Django.1.9, the username was restricted to 30 characters. For anonymous users, django-SHOP uses the session_key as a unique identifier. This required to encode the session_key of length 32 to a shorter variant, using base64 encoding.

In Django-1.10 the length of the username increased to 150 characters. Hence these encoding and decoding steps are not required any more and can be removed from the code.