Remove unnecessary footnote normalization from GLFM snapshot examples

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241 - closed)

Removes unnecessary normalization of the 07_01__gitlab_specific_markdown__footnotes__001 GLFM snapshot example, and instead uses the GITLAB_TEST_FOOTNOTE_ID ENV var support which is already used to generate the examples in scripts/lib/glfm/update_example_snapshots.rb

This leaves no normalizations in glfm_specification/input/gitlab_flavored_markdown/glfm_example_normalizations.yml, but we will likely need to add some more in A03. Add metadata support to GLFM example snaps... (!92507 - merged)

See also docs on update-example-snapshots.rb script: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#update-example-snapshotsrb-script

See the GLFM Specification Guide development documentation for much more context on all of this work: https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide

How to set up and validate locally

1. Run scripts/glfm/update-example-snapshots.rb and ensure there are no failures or changes (expected changes to *.yml files may occur after a rebase).
2. Running FOCUSED_MARKDOWN_EXAMPLES=07_01__gitlab_specific_markdown__footnotes__001 rspec spec/requests/api/markdown_snapshot_spec.rb should fail without the normalization or the ENV var set, but pass with the ENV var set.

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