[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/gigablast.py

@@ -24,7 +24,6 @@ about = {
 categories = ['general']
 # gigablast's pagination is totally damaged, don't use it
 paging = False
-language_support = True
 safesearch = True
 
 # search-url