pythonPackages.wxpython: drop SDL dependency (#391240)

this dependency was unused:

```
$ nix why-depends .#python313Packages.wxpython .#SDL --precise --all
these 5 paths will be fetched (4.96 MiB download, 21.60 MiB unpacked):
/nix/store/vhz0cn55wadyknv3bz4in7drm0mmpny3-SDL_compat-1.2.68
/nix/store/jfia42fgcpz2gmb5l1mqps5qaaan3lcn-libadwaita-1.6.4
/nix/store/555981fqc4rkjdfapzsw092bqyrbi5s3-sdl2-compat-2.32.52
/nix/store/kq5gmdxv139qk1j50grxlqvrsrlx9aq5-sdl3-3.2.6-lib
/nix/store/d9vlzcqg021zzyw6vwc7dfin8lbm86a1-zenity-4.0.5
'git+file:///home/grimmauld/nixpkgs#python313Packages.wxpython' does not depend on 'git+file:///home/grimmauld/nixpkgs#SDL'
```

```
$ nix why-depends .#python313Packages.wxpython .#SDL --derivation --all
/nix/store/79x3l16rh508qmbprz7xslqvd568gd8z-python3.13-wxpython-4.2.2.drv
└───/nix/store/s0kxrg633g9j5yvqiw1d2dgw76hg5r53-SDL_compat-1.2.68.drv
```

pkgs/development/python-modules/wxpython/4.2.nix

···
   libglvnd,
   libgbm,
   pango,
-  SDL,
   webkitgtk_4_0,
   wxGTK,
   xorgproto,
···
     attrdict
     pkg-config
     setuptools
-    SDL
     sip
     which
     wxGTK
···
   buildInputs =
     [
       wxGTK
-      SDL
     ]
     ++ lib.optionals stdenv.hostPlatform.isLinux [
       gst_all_1.gst-plugins-base
···
 
     export DOXYGEN=${doxygen}/bin/doxygen
     export PATH="${wxGTK}/bin:$PATH"
-    export SDL_CONFIG="${lib.getExe' (lib.getDev SDL) "sdl-config"}"
     export WAF=$PWD/bin/waf
 
     ${python.pythonOnBuildForHost.interpreter} build.py -v --use_syswx dox etg sip --nodoc build_py