[fix] bing engine

before this commit, sometimes there are no results
use a generic user-agent instead of one with the OS "Windows NT 6.3; WOW64"

searx/engines/bing.py

@@ -47,8 +47,6 @@ def request(query, params):
 
     params['url'] = base_url + search_path
 
-    params['headers']['User-Agent'] = gen_useragent('Windows NT 6.3; WOW64')
-
     return params