[system-health] Remove booting stage in system health service

[Junchao-Mellanox]

What I did

For SYSTEM READY feature. Currently, there is a booting stage in system health service to indicate that the system is loading SONiC component. This booting stage is no longer needed because SYSTEM READY feature will treat that stage as system "NOT READY".

How I did it

1. Remove booting stage
2. Adjust unit test cases

How to verify it

1. Manual test
2. Unit test
3. sonic-mgmt Regression

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[sg893052]

Looks fine.
As per system ready, the operational data(service status and appready status) will be stored in STATE_DB.
We may have to come up with an option in the CLI to show the services and its status to fetch the data from STATE_DB.

[liat-grozovik]

approved following the reviewers approval