python: Get rid of some unused cruft in main.py.
python: Get rid of some unused cruft in main.py.

There was some code at the end of main.py which would let you run it
directly. This would parse options passed to the script, and show you
what they equaled.

Also, the "main" function would optionally let you pass in options to
override whatever it would find with parse_arguments. This is no longer
used.

Change-Id: Ib0effa7ac2b4a51b68994372a7c7fcf1c9b4dc13
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50707
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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