[3.12] gh-108303: Remove unused Lib/test/sgml_input.html (GH-108305) (#109671)

gh-108303: Remove unused Lib/test/sgml_input.html (GH-108305)

In Python 2.7, the file was used by Lib/test/test_sgmllib.py to test
Lib/sgmllib.py. The sgmllib module and its tests have been removed in
Python 3.0.
(cherry picked from commit d2879f2095abd5c8186c7f69c964a341c2053572)

Co-authored-by: Victor Stinner <vstinner@python.org>