Installer: separate writing bootsector from other install finishing code

Remove the InstallerInitScript (it does nothing) and the
InstallerFinishScript (it does too many things). Instead implement the
finishing directly in Installer. Separate writing the bootsector, so
that the "write bootsector" menu writes only the bootsector.

Fixes #16303

data/system/boot/InstallerFinishScript

@@ -1,21 +0,0 @@
-#!/bin/sh
-## The installer finish script.
-
-target=$1
-
-if [ -z "$target" ]; then
-	echo "Usage: $0 <volume>"
-	exit 1
-fi
-
-if [ ! -d "$target" ]; then
-	echo "$target isn't mounted"
-	exit 1
-fi
-
-mkdir -p "$target/system/cache/tmp"
-
-# remove Installer link
-rm -f "$target/home/Desktop/Installer"
-
-makebootable "$target"

data/system/boot/InstallerInitScript

@@ -1,5 +0,0 @@
-#!/bin/sh
-## The installer init script.
-
-
-