Fix instances of "Expression has no effect" warning in SessionService.js

[jvigliotta]

Describe the defect

We have 3 instances where we have expressions that have no effect in SessionService.js. Two are legitimate bugs regarding removing listeners for realtime and historical sessions.

• https://github.com/NASA-AMMOS/openmct-mcws/security/code-scanning/37 (return the expression to the filter method)
• https://github.com/NASA-AMMOS/openmct-mcws/security/code-scanning/38 (return the expression to the filter method)
• Expression has no effect (set to null in constructor)

Expected behavior

Steps To Reproduce

1. This is code based, it leaves listeners for ever session

Additional context