remove old year level hint.css

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

Update year level `.filelist` accordingly.

2018/.filelist

@@ -5,7 +5,6 @@
 2018/.filelist
 2018/.gitignore
 2018/guidelines.txt
-2018/hint.css
 2018/index.html
 2018/iocccsize.c
 2018/iocccsize.mk

2018/hint.css

@@ -1,77 +0,0 @@
-* {
-    font-family: Helvetica Neue, Helvetica, Arial, sans-serif;
-}
-body {
-    max-width: 50em;
-    margin: 0 auto;
-    color: #111;
-}
-
-pre, code {
-    font-family: Monaco, Courier New, monospace;
-    font-size: 12px;
-}
-
-h1 {
-    font-size: 40px;
-    font-weight: bold;
-    letter-spacing: -1px;
-    margin-bottom: -5px;
-}
-h1 code {
-    font-size: 40px;
-}
-
-h2 {
-    color: rgb(43,105,145);
-    font-weight: bold;
-    margin-bottom: -5px;
-}
-h2 code {
-    font-size: 22px;
-}
-
-h3 {
-    /* color: rgb(43,105,145); */
-    font-weight: bold;
-    margin-bottom: -5px;
-}
-h3 code {
-    font-size: 16px;
-}
-
-a {
-    color: blue;
-    text-decoration: underline;
-}
-a:visited {
-    color: navy;
-}
-a:hover {
-    text-decoration: underline;
-}
-
-pre {
-    border-radius: 10px;
-    -webkit-border-radius: 10px;
-    -moz-border-radius: 10px;
-    box-shadow: 0px 2px 8px rgba(0, 0, 0, 0.3);
-    -webkit-box-shadow: 0px 2px 8px rgba(0, 0, 0, 0.3);
-    -moz-box-shadow: 0px 2px 8px rgba(0, 0, 0, 0.3);
-    border-width: 1px;
-    border-color: rgb(16,48,0);
-    border-style: solid;
-    padding: 0.6em;
-    padding-left: 1em;
-    margin-left: 5%;
-    margin-right: 8%;
-    background-color: rgb(0,47,53);
-    overflow: auto;
-    color: rgb(181,199,255);
-}
-
-p, li {
-    font-size: 14px;
-    line-height: 18px;
-}
-