Do not include symbols of irrelevant categories in AST

This will simplify the code as it won't need to filter out the
irrelevant symbols.
Get rid of now useless |variables| and |functions| methods

The |variables| and |functions| methods defined on some YCP AST nodes
were always used in tandem. Now that we filtered out irrelevant symbols
from the AST (see previous commit), we can just use |symbols| instead.