[mod] searx.search.EngineRef: remove from_bang parameter

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.