remove Mambaforge variant from CI

It is being sunsetted and will stop working entirely in Jan 2025.

Ref: https://conda-forge.org/news/2024/07/29/sunsetting-mambaforge/

Author: Michael-J-Ward

.github/workflows/conda.yml

@@ -70,7 +70,6 @@ jobs:
       - name: Set up Python
         uses: conda-incubator/[email protected]
         with:
-          miniforge-variant: Mambaforge
           use-mamba: true
           python-version: "3.8"
           channel-priority: strict