[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 d2879f2)

Co-authored-by: Victor Stinner <[email protected]>

Author: miss-islington