Removed invalid structure parameters

[alexisbalbachan]

Related to #1876
Several smb structures contain a FileNameLength field which is specified by 3 parameters. However for the third parameter to be ever used, the second parameter has to have a specific syntax, this was not the case.
This PR removes the third parameter from those fields, it shouldn't have any splash damage as they were not being used

Additional Info
structure.py specifies that formats with a '_' in their values will use a third parameter:

impacket/impacket/structure.py

Line 73 in ae0ec30

_ will not pack the field. Accepts a third argument, which is an unpack code. See _Test_UnpackCode for an example

however all the offending structures have the following format '?-field' with ? being any struct size specifier (B|H|L):

impacket/impacket/structure.py

Line 69 in ae0ec30

?-field length of field named 'field', formatted as specified with ? ('?' may be '!H' for example). The input value overrides the real length

so they don't need a third parameter.

If we inspect structure.py, the only time were functions are being evaluated is when format either contains '_' or '=', it wasn't the case in the modified structures:

impacket/impacket/structure.py

Lines 203 to 210 in ae0ec30

	two = format.split('=')
	if len(two) >= 2:
	try:
	return self.pack(two[0], data)
	except:
	fields = {'self':self}
	fields.update(self.fields)
	return self.pack(two[0], eval(two[1], {}, fields))

impacket/impacket/structure.py

Lines 307 to 313 in ae0ec30

	if format[:1] == '_':
	if dataClassOrCode != b:
	fields = {'self':self, 'inputDataLeft':data}
	fields.update(self.fields)
	return eval(dataClassOrCode, {}, fields)
	else:
	return None