Cleanup dead resetFrame code

[alexrudd2]

mypy flags the else branch as unreachable because the isinstance check always passes. I think this happened in ce67df9 but went unnoticed because the function isn't annotated.

pymodbus/pymodbus/server/async_io.py

Lines 162 to 172 in 214c7ed

	if isinstance(self, ModbusServerRequestHandler):
	Log.error(
	'Unknown exception "{}" on stream {} forcing disconnect',
	exc,
	self.comm_params.comm_name,
	)
	self.close()
	self.callback_disconnected(exc)
	else:
	Log.error("Unknown error occurred {}", exc)
	reset_frame = True # graceful recovery

So this PR deletes the entire conditional, as well as the now-useless reset_frame flag.

Also, it cleans up the docstring mangled by 18aa463

[janiversen]

LGTM, thanks