python,python3: remove `--with-system-ffi=no` config arg

python,python3: remove `--with-system-ffi=no` config arg

Report https://github.com/openwrt/packages/issues/5638
It was mentioned that this causes build failures on Mac OS X.

The default behavior [in the setup.py script] is to check whether
`--with-system-ffi` is present in the CONFIG_ARGS env var.

However that back-fires a bit when `--with-system-ffi=no`, because the
condition `not '--with-system-ffi' in sysconfig.get_config_var("CONFIG_ARGS")`
evaluates to true.

This is a small bug in the `setup.py` script, but it looks like the
easiest/cleanest way to address it on our end is to just remove it entirely
from the HOST_CONFIGURE_ARGS.
At least that's how it looks like when testing on a Linux machine.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>