Build with the imported IrrlichtMt at irr

Also remove the now useless options (like IRRLICHT_INCLUDE_DIR)
and update download instructions, CI and similar.

Co-authored-by: sfan5 <sfan5@live.de>

.github/workflows/windows.yml

@@ -11,7 +11,6 @@ on:
       - '**/CMakeLists.txt'
       - 'cmake/Modules/**'
       - 'util/buildbot/**'
-      - 'misc/irrlichtmt_tag.txt'
       - 'misc/*.manifest'
       - '.github/workflows/windows.yml'
   pull_request:
@@ -23,7 +22,6 @@ on:
       - '**/CMakeLists.txt'
       - 'cmake/Modules/**'
       - 'util/buildbot/**'
-      - 'misc/irrlichtmt_tag.txt'
       - 'misc/*.manifest'
       - '.github/workflows/windows.yml'
 
@@ -97,11 +95,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Checkout IrrlichtMt
-        run: |
-          $ref = @(Get-Content misc\irrlichtmt_tag.txt)
-          git clone https://github.com/minetest/irrlicht lib\irrlichtmt --depth 1 -b $ref[0]
-
       - name: Restore from cache and run vcpkg
         uses: lukka/run-vcpkg@v7
         with: