improve approach to redundant slash redirection

Since merge_slashes gets rid of the redundant slashes, a no-op rewrite
will get rid of all of them at once instead of requiring one redirect to
get rid of each redundant slash.

nginx/nginx.conf

@@ -192,8 +192,8 @@ http {
         gzip_static on;
         brotli_static on;
 
-        if ($request_uri ~ (.*)//(.*)) {
-            return 301 $1/$2;
+        if ($request_uri ~ //) {
+            rewrite ^(.*)$ $1 permanent;
         }
 
         location = /, {