remove struts.freemarker.mru.max.strong.size from default.properties

if struts.freemarker.mru.max.strong.size > 0 ,freemarker will use MruCacheStorage instead of SoftCacheStorage,and MruCacheStorage is not concurrent,it will synchronized on every call,it will affect performance.

in TemplateCache:

CachedTemplate cachedTemplate;
if(isStorageConcurrent) {
    cachedTemplate = (CachedTemplate)storage.get(tk);
}
else {
    synchronized(storage) {
       cachedTemplate = (CachedTemplate)storage.get(tk);
    }
}

please remove struts.freemarker.mru.max.strong.size from default.properties lets developers make their own choice.
a better solution is implements a concurrent MruCacheStorage for struts2