Revert "Provide our own SDL2 dll files"

This reverts commit 91530c61ad.

i hate windows, i hate sdl breaking with every single update

libs/SDL2/SRB2NOTE.txt

@@ -13,8 +13,3 @@ https://www.libsdl.org/download-2.0.php
 
 * i686-w64-mingw32
 * x86_64-w64-mingw32
-
-# SRB2Kart Saturn note:
-Due to a good amount of issues with newer SDL releases, we decided to ship our own Version of the library.
-This version is just the 2.28.5 release with a fix backported for non US keyboard layouts that prevented the right ALT GR key from working properly.
-It can be found under: https://github.com/alufolie91/SDL/tree/backportaltgrfix