Investigate removal of initial update check without registered clients

[sgraband]

Feature Description:

With the update check no longer triggered on backend startup, the code path in theia-updater-impl.ts that guards against update checks without registered clients (initialCheck handling) may be obsolete. Investigate whether this situation can still occur. If not, simplify the implementation by removing the dead/defensive code.

See https://github.com/eclipse-theia/theia-ide/pull/638/changes/BASE..f6707ee4f20ff364a59e923e9fb338a64eecbc3d#r2694630678