Remove support for accepting raw session token in /password/change/finish

[vbudhram]

This is a follow up to mozilla/fxa-content-server#3875

Once this has been able to ride the trains, remove support for accepting session data from /password/change/finish

[rfk]

This is blocked on the above content-server PR

[vladikoff]

content server 3875

[shane-tomlinson]

@vbudhram - the content server portion of this has merged. Can this be pulled out of blocked?

[vbudhram]

@shane-tomlinson Yup, should be good to work on this train.