[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/doku.py

@@ -20,7 +20,6 @@ about = {
 # engine dependent config
 categories = ['general']  # TODO , 'images', 'music', 'videos', 'files'
 paging = False
-language_support = False
 number_of_results = 5
 
 # search-url