checksums: Remove or fix up unfetchable or unverifiable md5 versions

This commit either removes remaining md5 checksums, or it replaces them
with sha256 checksums verified by fetching over https.

var/spack/repos/builtin/packages/libffi/package.py

@@ -15,9 +15,6 @@ class Libffi(AutotoolsPackage):
 
     version('3.2.1', sha256='d06ebb8e1d9a22d19e38d63fdb83954253f39bedc5d46232a05645685722ca37',
             url="https://www.mirrorservice.org/sites/sourceware.org/pub/libffi/libffi-3.2.1.tar.gz")
-    # version('3.1', md5='f5898b29bbfd70502831a212d9249d10',url =
-    # "ftp://sourceware.org/pub/libffi/libffi-3.1.tar.gz") # Has a bug
-    # $(lib64) instead of ${lib64} in libffi.pc
 
     @property
     def headers(self):