Removal of critical Association for functional FRU

[GiridhariKrishnan]

When a faulty CEC FRU is repaired or replaced during runtime,
its corresponding chassis critical associations were left unremoved.
This leads in displaying Chassis Health Rollup as "Critical" in GUI even
after the fault FRU is repaired/replaced.

This commit has changes that removes the critical association on chassis,
when the FRU's "Functional" property (present under the d-bus interface
"xyz.openbmc_project.State.Decorator.OperationalStatus") is changed from
false to true, which indicates that the FRU is repaired and is functional.

This is needed to handle hotpluggable or concurrently maintainable FRUs
which are associated with the chassis.