[fix] xpath, mojeek: fix commit 58d72f26925d56e22330c54be03c3dcbee0c4135

before commit 58d72f2, category was not set in xpath.py,
so searx/engines/__init__py was setting the category to ['general']

the commit 58d72f2 set the category to [] which is not replaced by searx/engines/__init__.py
consequence: the mojeek engine is hidden in the preferences.

this commit revert the xpath.py change.

close #2368

searx/engines/xpath.py

@@ -7,7 +7,6 @@ url_xpath = None
 content_xpath = None
 title_xpath = None
 thumbnail_xpath = False
-categories = []
 paging = False
 suggestion_xpath = ''
 results_xpath = ''
@@ -39,7 +38,7 @@ def request(query, params):
 def response(resp):
     results = []
     dom = html.fromstring(resp.text)
-    is_onion = True if 'onions' in categories else False
+    is_onion = True if 'onions' in categories else False  # pylint: disable=undefined-variable
 
     if results_xpath:
         for result in eval_xpath_list(dom, results_xpath):