Update argparse.py: remove unused metavar

[WrRan]

remove unused metavar of _StoreConstAction.__init__

[savannahostrowski]

Removing metavar here results in a NameError because it's expected later on in add_argument. If you try to build, you'll get:

Traceback (most recent call last):
  File "/Users/savannah/cpython/./Tools/build/generate-build-details.py", line 192, in <module>
    main()
    ~~~~^^
  File "/Users/savannah/cpython/./Tools/build/generate-build-details.py", line 163, in main
    parser.add_argument('location')
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/savannah/cpython/Lib/argparse.py", line 1455, in add_argument
    action = action_class(**kwargs)
  File "/Users/savannah/cpython/Lib/argparse.py", line 919, in __init__
    metavar=metavar,
            ^^^^^^^
NameError: name 'metavar' is not defined

[savannahostrowski]

Closing this PR as metavar is not unused