[mod] dynamically set language_support variable

The language_support variable is set to True by default,
and set to False in only 5 engines.

Except the documentation and the /config URL, this variable is not used.

This commit remove the variable definition in the engines, and
set value according to supported_languages length: False when the length is 0,
True otherwise.

Close #2485

searx/engines/yahoo_news.py

@@ -25,7 +25,6 @@ about = {
 # engine dependent config
 categories = ['news']
 paging = True
-language_support = True
 
 # search-url
 search_url = 'https://news.search.yahoo.com/search?{query}&b={offset}&{lang}=uh3_news_web_gs_1&pz=10&xargs=0&vl=lang_{lang}'  # noqa