from_bang is True when the user query contains a bang.
In this case the category is also set to 'none'.
from_bang only usage was in searx.webadapter.parse_specific :
if from_bang is True, then the EngineRef category is ignored and force to 'none'.
This commit also removes the searx.webadapter.parse_sepecific function.
see searx.search.processors.abstract.EngineProcessor
First the method searx call the get_params method.
If the return value is not None, then the searx call the method search.
