internal/app: remove app interface

It is very clear at this point that there will not be multiple implementations of App, and the internal/app package will never move out of internal due to hsu.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/app.go

@@ -4,22 +4,8 @@ package app
 import (
 	"syscall"
 	"time"
-
-	"hakurei.app/hst"
-	"hakurei.app/internal/app/state"
 )
 
-type App interface {
-	// ID returns a copy of [state.ID] held by App.
-	ID() state.ID
-
-	// Seal determines the outcome of config as a [SealedApp].
-	// The value of config might be overwritten and must not be used again.
-	Seal(config *hst.Config) (SealedApp, error)
-
-	String() string
-}
-
 type SealedApp interface {
 	// Run commits sealed system setup and starts the app process.
 	Run(rs *RunState) error