chore: MAAS as a snap only - drop install_type from websockets

[r00ta]

Done

• In the backend we removed install_type, as every controller is supposed to be running in a snap env in 4.0. So I removed install_type and all the logics around that
• updated the image source keyring path, as it'll be always the snap one

QA steps

• run the UI with the backend running this patch chore: MAAS as a snap only - drop debian packaging maas#504
• checked the image source page
• checked the controller page

[nehjoshi]

Looks good for the most part. There are still some trailing references to install_type to remove:

• src/app/store/controller/types/enum.ts: Since we only support snap, we can remove the enum entirely as it leaves only 1 option

• src/app/store/controller/types/base.ts: Remove install_type from ControllerVersions

• src/app/controllers/views/ControllerList/components/VersionColumn/VersionColumn.tsx: Conditionally renders controller install type

• src/app/base/components/node/OverviewCard/ControllerStatusCard/ControllerStatusCard.tsx: We can display the snap channel directly without checking if install_type==='deb'.

• Some references to install_type can be removed from src/app/base/components/node/OverviewCard/ControllerStatusCard/ControllerStatusCard.test.tsx

[r00ta]

Thanks! I addressed the comments and re-tested

[nehjoshi]

LGTM!