[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/youtube_api.py

@@ -21,7 +21,6 @@ about = {
 # engine dependent config
 categories = ['videos', 'music']
 paging = False
-language_support = True
 api_key = None
 
 # search-url