[workflows] delete ilmbase dependency in windows workflow

`ilmbase` is no longer offered:
* https://github.com/msys2/MINGW-packages/pull/12871#issuecomment-1235816186
* https://packages.msys2.org/package/mingw-w64-ucrt-x86_64-ilmbase

.github/workflows/windows_test.yml

@@ -30,7 +30,6 @@ jobs:
             git
             mingw-w64-ucrt-x86_64-cmake
             mingw-w64-ucrt-x86_64-doctest
-            mingw-w64-ucrt-x86_64-ilmbase
             mingw-w64-ucrt-x86_64-openimageio
             mingw-w64-ucrt-x86_64-libdeflate
             mingw-w64-ucrt-x86_64-libunistring