build: drop the pkg-config requirement from the Windows build (#15469)

The FFmpeg camera view port made pkg-config a required build tool on
Windows. Windows does not ship one, so every Windows developer has to
install it before the build will configure:

  Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
  Call Stack (most recent call first):
    CMakeLists.txt:480 (find_package)

Nothing on Windows needs it. FFmpeg there is a prebuilt zip unpacked
into the deps prefix, whose DLLs the top level CMakeLists already names
by exact soname. The version is fixed before configure runs, so
find_library against that prefix does the job, as on macOS.

Also drops the CI step that installed pkg-config, gated on !SELF_HOSTED
so it never ran on self-hosted runners, and re-comments the if(WIN32)
block that #15234 uncommented only for that find_package.

.github/workflows/build_orca.yml

@@ -385,13 +385,6 @@ jobs:
           dir "C:/Program Files (x86)/Windows Kits/10/Include"
           choco install nsis
 
-      - name: Install pkg-config
-        # FFmpeg is discovered via pkg-config (pkg_check_modules LIBAV in
-        # src/slic3r/CMakeLists.txt); the Windows runners don't ship it.
-        if: runner.os == 'Windows' && !vars.SELF_HOSTED
-        run: |
-          choco install pkgconfiglite -y
-
       - name: Build slicer Win
         if: runner.os == 'Windows'
         working-directory: ${{ github.workspace }}