build-scripts: Removed winrtbuild.*, no longer used.

WinRT/UWP is still supported, but you have to use the VS2019
project files, now.

Fixes #5639.

docs/release_checklist.md

@@ -11,7 +11,6 @@
         `CFBundleVersion`
     * `Makefile.os2`: `VERSION`
     * `Makefile.w32`: `*_VERSION`
-    * `build-scripts/winrtbuild.ps1`: `$SDLVersion`
     * `include/SDL_version.h`: `SDL_*_VERSION`, `SDL_PATCHLEVEL`
     * `src/main/windows/version.rc`: `FILEVERSION`, `PRODUCTVERSION`,
         `FileVersion`, `ProductVersion`