remove `Subpage` page component

[arturasmckwcz]

Description

in App.tsx all routes above line #144 (excluding root "/") calls Subpage.tsx page component, which was the layout component for other pages. now it's obsolete.
But not all called components are obsolete, some of them are re-used by the router in App.tsx.

this includes:

1. remove Subpage.tsx
2. remove not re-used components it calls