Revert Windows MKL - not supported via pkg-config on conda

MKL on Windows conda doesn't provide pkg-config files, only CMake
modules which fail to detect with our current setup. Reverting both
Windows matrix jobs to OpenBLAS.

We still have good BLAS backend coverage:
- Linux: OpenBLAS (apt) + MKL (conda dedicated workflow)
- macOS: Accelerate framework
- Windows: OpenBLAS (all jobs)

This reverts part of commit 63f571a.

Author: ilayn

.github/workflows/test-matrix.yml

@@ -34,14 +34,8 @@ jobs:
       run: |
         conda install -y meson ninja numpy pytest openblas pkg-config
 
-    - name: Install dependencies (Windows - Python 3.12 with MKL)
-      if: runner.os == 'Windows' && matrix.python-version == '3.12'
-      shell: bash -l {0}
-      run: |
-        conda install -y meson ninja numpy pytest mkl mkl-devel clang llvm-tools lld pkg-config
-
-    - name: Install dependencies (Windows - Python 3.13 with OpenBLAS)
-      if: runner.os == 'Windows' && matrix.python-version == '3.13'
+    - name: Install dependencies (Windows)
+      if: runner.os == 'Windows'
       shell: bash -l {0}
       run: |
         conda install -y meson ninja numpy pytest openblas clang llvm-tools lld pkg-config