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Mention that flake8 and black can be run via Makefile targets.

Remove the outdated list of flake8 warnings.  The Makefile ignores more
warnings/errors, and it doesn't seem to be useful to duplicate this list
here.

Also remove the description of the black tool's arguments for the same
reason.

Change-Id: I941c3ab68b7a3d2477f7fbb5d28603987d0b2cab
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>