[xenforo] implement generic XenForo forum extractors

support
- https://simpcity.cr/
- https://nudostar.com/forum/ (#8333)

gallery_dl/extractor/__init__.py

@@ -146,7 +146,6 @@ modules = [
     "nozomi",
     "nsfwalbum",
     "nudostar",
-    "nudostarforum",
     "okporn",
     "paheal",
     "patreon",
@@ -187,7 +186,6 @@ modules = [
     "senmanga",
     "sexcom",
     "shimmie2",
-    "simpcity",
     "simplyhentai",
     "sizebooru",
     "skeb",
@@ -235,6 +233,7 @@ modules = [
     "wikifeet",
     "wikimedia",
     "xasiat",
+    "xenforo",
     "xfolio",
     "xhamster",
     "xvideos",