Drop collation description cache.

It really does not improve performance much and it just consumes memory.
Anyway on most pages this function is called just once for each
collation, so all it does is filling up the cache but never using it.

libraries/mysql_charsets.lib.php

@@ -198,14 +198,6 @@ function PMA_getServerCollation() {
  * @return  string  collation description
  */
 function PMA_getCollationDescr($collation) {
-    static $collation_cache;
-
-    if (!is_array($collation_cache)) {
-        $collation_cache = array();
-    } elseif (isset($collation_cache[$collation])) {
-        return $collation_cache[$collation];
-    }
-
     if ($collation == 'binary') {
         return __('Binary');
     }