Remove no longer relevant text about passivation from Externalize Sessions section

The section describing steps needed to Externalize Sessions from EAP to JDG (https://access.redhat.com/documentation/en-us/red_hat_data_grid/7.2/html/administration_and_configuration_guide/externalize_sessions#externalize_http_session_from_jboss_eap_to_jboss_data_grid) appears to need an update.

It looks like the required configuration in jboss-web.xml correctly removed the no longer supported passivation-config, but th e text surrounding the code snippet still mentions it:
"Add passivation and cache information..."
and
"NOTE
The passivation timeouts above are provided..."