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Remove the outdated list of flake8 warnings.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>