Remove unused libffi build script (#965)

The compile-time cryptography step that used to use the libffi archives
on S3 was removed in 2018:
https://github.com/heroku/heroku-buildpack-python/commit/c373e80c1285260e5adcbc855f54bbeb6999005c

...since the `cryptography` Python package now ships wheels.

The script is also incorrect, since similar to #964 it only skips builds
for Heroku-16, whereas all stacks since Cedar-14 include libffi-dev in
the build image, so don't need it built/uploaded for later vendoring.

Refs W-7485877.

builds/libraries/vendor/libffi

@@ -1,28 +0,0 @@
-#!/usr/bin/env bash
-# Build Path: /app/.heroku/vendor/
-
-OUT_PREFIX=$1
-
-# Skip the build for heroku-16.
-if [[ $S3_PREFIX == "heroku-16" ]]; then
-    exit 0
-fi
-
-# Use new path, containing autoconf.
-export PATH="/app/.heroku/python/bin/:$PATH"
-hash -r
-
-
-echo "Building libffi…"
-
-SOURCE_TARBALL='https://github.com/libffi/libffi/archive/v3.1.tar.gz'
-
-curl -L $SOURCE_TARBALL | tar x
-
-cd v3.1
-./configure --prefix=$OUT_PREFIX --disable-static &&
-make
-make install
-
-# Cleanup
-cd ..