[chore] engines: remove redundant usages of utils#gen_useragent

These engines override the user agent manually using `gen_useragent`, although that's already done in the online preprocessor that runs before the actual `request(query, params)` method is called. Hence, this call is duplicated.

Related:
- https://github.com/searxng/searxng/pull/4990#discussion_r2195142838

searx/engines/uxwing.py

@@ -4,7 +4,7 @@
 from urllib.parse import quote_plus
 from lxml import html
 
-from searx.utils import eval_xpath, eval_xpath_list, extract_text, gen_useragent
+from searx.utils import eval_xpath, eval_xpath_list, extract_text
 
 about = {
     "website": 'https://uxwing.com',
@@ -22,7 +22,6 @@ enable_http2 = False
 
 def request(query, params):
     params['url'] = f"{base_url}/?s={quote_plus(query)}"
-    params['headers'] = {'User-Agent': gen_useragent()}
     return params