Remove redundant --with-system-ffi configure option

Since as of Python 3.7+ using system `ffi` is now mandatory (and so the default)
on Linux:
https://bugs.python.org/issue27979
python/cpython@f40d4dd

Removing the option resolves this configure warning:

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

Closes docker-library#715.

Author: edmorley

3.10/alpine3.14/Dockerfile

@@ -159,7 +159,6 @@ RUN set -eux; \
 		--with-lto \
 {{ ) end -}}
 		--with-system-expat \
-		--with-system-ffi \
 		--without-ensurepip \
 	; \
 	nproc="$(nproc)"; \