remove old year level hint.css

Remove, for IOCCC years 2011 thru 2020, the `hint.css` files.

Update year level `.filelist` accordingly.

2019/.filelist

@@ -5,7 +5,6 @@
 2019/.filelist
 2019/.gitignore
 2019/guidelines.txt
-2019/hint.css
 2019/index.html
 2019/iocccsize.c
 2019/iocccsize.mk