remove defaults channel from conda

Author: Moritz-Alexander-Kern

.github/workflows/CI.yml

@@ -167,7 +167,7 @@ jobs:
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
           mamba-version: "*"
-          channels: conda-forge,defaults
+          channels: conda-forge
           channel-priority: true
           activate-environment: elephant
           environment-file: requirements/environment-tests.yml
@@ -388,7 +388,7 @@ jobs:
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
           mamba-version: "*"
-          channels: conda-forge,defaults
+          channels: conda-forge
           channel-priority: true
           activate-environment: elephant
           environment-file: requirements/environment-tests.yml