[exhentai] improve image limits check

- check if current image is the '509 Bandwidth Exceeded' notification
  (https://ehgt.org/g/509.gif or https://exhentai.org/img/509.gif)
- remove 'limits' option

docs/configuration.rst

@@ -922,20 +922,6 @@ Description
     * ``"exhentai.org"``: Use ``exhentai.org`` for all URLs
 
 
-extractor.exhentai.limits
--------------------------
-Type
-    ``bool`` or ``integer``
-Default
-    ``true``
-Description
-    Check image download limits
-    and stop extraction when they are exceeded.
-
-    If this value is an ``integer``, it gets used as the limit maximum
-    instead of the value listed on ``https://e-hentai.org/home.php``
-
-
 extractor.exhentai.metadata
 ---------------------------
 Type