Remove unnecessary OptionParser statements.

Addresses
https://github.com/jensengroup/propka-3.1/pull/29#discussion_r424418158.

propka/lib.py

@@ -224,14 +224,9 @@ def loadOptions(*args):
     Returns:
         argparse namespace
     """
-    from optparse import OptionParser
-
     # defining a 'usage' message
     usage = "usage: %prog [options] filename"
 
-    # creating a parser
-    parser = OptionParser(usage)
-
     # loading the parser
     parser = build_parser()