python3 - drop use use of '--with-system-ffi' flag.

[smoser]

Being explicit is good, but in the event that it generates a confusing WARNING message it is not so good.

The python3.10 and 3.11 logs would show:

checking for --with-system-ffi... yes
configure: WARNING: --with(out)-system-ffi is ignored on this platform

Python 3.12 and python 3.13 logs show:

configure: WARNING: unrecognized options: --with-system-ffi

system-ffi was made default in 3.7 and then later simply required. https://bugs.python.org/issue27979

[smoser]

For posterity, this log was copied from pre-commit build of #28709.
job-logs.txt

$ grep -E -A2 '(melange is building|system-ffi)' /tmp/job-logs.txt 
2024-09-18T15:47:31.9872832Z 2024/09/18 15:47:31 INFO melange is building:
2024-09-18T15:47:31.9873646Z 2024/09/18 15:47:31 INFO   configuration file: python-3.10.yaml
2024-09-18T15:47:31.9874906Z 2024/09/18 15:47:31 INFO   workspace dir: /temp/melange-workspace-2193452916
--
2024-09-18T15:47:57.4339989Z 2024/09/18 15:47:57 INFO checking for --with-system-ffi... yes
2024-09-18T15:47:57.4340829Z 2024/09/18 15:47:57 WARN configure: WARNING: --with(out)-system-ffi is ignored on this platform
2024-09-18T15:47:57.4355429Z 2024/09/18 15:47:57 INFO checking for --with-system-libmpdec... yes
2024-09-18T15:47:57.4356659Z 2024/09/18 15:47:57 INFO checking for --with-decimal-contextvar... yes
--
2024-09-18T15:53:58.0163070Z 2024/09/18 15:53:58 INFO melange is building:
2024-09-18T15:53:58.0163955Z 2024/09/18 15:53:58 INFO   configuration file: python-3.11.yaml
2024-09-18T15:53:58.0165081Z 2024/09/18 15:53:58 INFO   workspace dir: /temp/melange-workspace-1754456204
--
2024-09-18T15:54:21.4205450Z 2024/09/18 15:54:21 INFO checking for --with-system-ffi... yes
2024-09-18T15:54:21.4206248Z 2024/09/18 15:54:21 WARN configure: WARNING: --with(out)-system-ffi is ignored on this platform
2024-09-18T15:54:21.4220142Z 2024/09/18 15:54:21 INFO checking for --with-system-libmpdec... yes
2024-09-18T15:54:21.4221131Z 2024/09/18 15:54:21 INFO checking for --with-decimal-contextvar... yes
--
2024-09-18T16:03:38.0778395Z 2024/09/18 16:03:38 INFO melange is building:
2024-09-18T16:03:38.0778966Z 2024/09/18 16:03:38 INFO   configuration file: python-3.12.yaml
2024-09-18T16:03:38.0779924Z 2024/09/18 16:03:38 INFO   workspace dir: /temp/melange-workspace-1343122815
--
2024-09-18T16:03:51.1551813Z 2024/09/18 16:03:51 WARN configure: WARNING: unrecognized options: --with-system-ffi
2024-09-18T16:03:51.1844335Z 2024/09/18 16:03:51 INFO checking for git... found
2024-09-18T16:03:51.1913587Z 2024/09/18 16:03:51 INFO checking build system type... x86_64-pc-linux-gnu
--
2024-09-18T16:04:17.1449885Z 2024/09/18 16:04:17 WARN configure: WARNING: unrecognized options: --with-system-ffi
2024-09-18T16:04:17.1450625Z 2024/09/18 16:04:17 INFO configure: creating Modules/Setup.local
2024-09-18T16:04:17.1451169Z 2024/09/18 16:04:17 INFO configure: creating Makefile

[imjasonh]

I've converted this to draft to avoid merging it while we're migrating post-submit.

Sorry for the delay.