website: remove history/git-history.jpg

The history/git-history.jpg file is supposed to be generated so we
don't want to track it in git.

This was broken by the commit 388c0ef3d0
("website: add history page of the GNU Boot git repositories.").

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

website/.gitignore

@@ -29,3 +29,6 @@
 /website.tar.gz
 /pages/footer.include
 /pages/footer-git-commit.include
+
+# History
+/history/git-history.jpg