PEP 550: Fix footnotes

[hugovk]

Fix these warnings by removing the orphaned references:

pep-0550.rst:1704: WARNING: Footnote [4] is not referenced.
pep-0550.rst:1718: WARNING: Footnote [11] is not referenced.
pep-0550.rst:1720: WARNING: Footnote [13] is not referenced.
pep-0550.rst:1722: WARNING: Footnote [14] is not referenced.
pep-0550.rst:1724: WARNING: Footnote [15] is not referenced.
pep-0550.rst:1726: WARNING: Footnote [16] is not referenced.

[4] was removed in 1b8728d ("PEP-0550 V4" #375)

And the others were removed in 09c05c8 ("pep-550: Full rewrite: v2." #344)

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Also:

• Remove redundant emacs metadata
• Use plaintext literal block for ASCII diagram
• Update link redirects/https

Preview

https://pep-previews--2751.org.readthedocs.build/pep-0550/

[CAM-Gerlach]

LGTM aside from one small comment, thanks @hugovk

Not worth renumbering the references on an old rejected PEP, I would think...