package/python3: drop unsupported system ffi option

The --with-system-ffi was removed back in Python-3.12.0, in upstream
commit [1].

From the Python 3.12 release notes:

  - gh-100540: Removed the ``--with-system-ffi`` ``configure`` option;
    ``libffi`` must now always be supplied by the system on all non-Windows
    platforms.  The option has had no effect on non-Darwin platforms for
    several releases, and in 3.11 only had the non-obvious effect of invoking
    ``pkg-config`` to find ``libffi`` and never setting
    ``-DUSING_APPLE_OS_LIBFFI``.  Now on Darwin platforms ``configure`` will
    first check for the OS ``libffi`` and then fall back to the same
    processing as other platforms if it is not found.

Buildroot includes such a Python 3.12.x version since commit [2].

When compiling python3 in Buildroot, the package configuration step
reports the warning:

    configure: WARNING: unrecognized options: [...] --with-system-ffi

The commit drops the now defunct option.

[1] 25590eb5de
[2] 76cd14167f

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
[Julien: add links in commit log]
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit a7a3621c0b)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

package/python3/python3.mk

@@ -206,7 +206,6 @@ PYTHON3_CONF_OPTS += \
 	--without-ensurepip \
 	--without-cxx-main \
 	--with-build-python=$(HOST_DIR)/bin/python3 \
-	--with-system-ffi \
 	--disable-pydoc \
 	--disable-test-modules \
 	--disable-tk \