Remove unused instance variable.
Remove unused instance variable.

This variable leads to confusion when reading the source code.
Now that the session expiration logic is handled through a SOAP
expection it no longer makes sense.

REF: https://github.com/sportngin/active_zuora/commit/3e910db1b113f22dd7409ce4c39e1529dcb4d772

bigcommerce-labs/active_zuora,sportngin/active_zuora,Invoca/active_zuora,jonnymacs/active_zuora