parsergen: remove symbol synthesis option.

parsergen: remove symbol synthesis option.

This idea never worked, and cannot work as we cannot
magically synthesis the ast node to go with a synthesized
symbol.
If we want to synthesize something on error, we just use
a production like
   foo -> ERRROR ${ $0 = a_new_for(); }$

Signed-off-by: NeilBrown <neil@brown.name>