CI: Revert "CI: Set MPLBACKEND=Agg on Windows"

This reverts commit 90dea77.

This commit did not actually solve the problem, as matplotlib on Windows
does not honor the environment variable set this way. It still used
TkAgg and failed sporadically.

.github/workflows/test.yml

@@ -41,11 +41,6 @@ jobs:
       - name: Setup testdata
         uses: "./.github/actions/setup_testdata"
 
-      # Windows sporadically fails on tkinter pyplots
-      - name: Set MPLBACKEND=Agg on Windows
-        if: ${{ matrix.os == 'windows-latest' }}
-        run: echo "MPLBACKEND=Agg" >> $GITHUB_ENV
-
       - name: Run tests
         run: pytest -n 4 tests --disable-warnings --generate-plots