Remove unused references to "environment file"

[hoechenberger]

Proposed documentation enhancement

mne-python/doc/install/advanced.rst

Line 244 in f1a8120

.. _environment file: https://raw.githubusercontent.com/mne-tools/mne-python/main/environment.yml

mne-python/doc/install/check_installation.rst

Line 87 in f1a8120

.. _environment file: https://raw.githubusercontent.com/mne-tools/mne-python/main/environment.yml