cmake: declutter root + remove obsolete variables

.editorconfig

@@ -22,7 +22,7 @@ indent_style = space
 insert_final_newline = true
 trim_trailing_whitespace = true
 
-[{cmake_uninstall.cmake.in,test/CMakeLists.txt,./SDL3Config.cmake.in}]
+[{cmake/cmake_uninstall.cmake.in,test/CMakeLists.txt,cmake/SDL3Config.cmake.in}]
 indent_size = 4
 indent_style = space
 insert_final_newline = true