fixed memory leak on minimized due to missing deletion of unused textures (fixes #14245)

I made this a pr since it looks like m_AppActive is also used for SDL and android.

Using multiimage controls lead to heavy memory leaking when being minimized because we don't free the unused textures anymore. I just removed the hole block because on windows doing a FreeUnusedTextures() inside that block costs more cpu power than process the whole Render path (7% to 5%).
Given that we just want to leave early to save resources this makes no sense at least on win. What I need to know is if this has counter effects on the other platforms using m_AppActive.