contrib: remove deprecated memfd-bind binary

This was a really ugly hack to try to reduce the impact of our original
set of CVE-2019-5736 mitigations, but unfortunately had too many caveats
to its use to ever be really useful. In addition, it was completely
obsoleted by the migration to using an detached overlayfs mount in
commit 515f09f7b1 ("dmz: use overlayfs to write-protect /proc/self/exe
if possible").

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

.gitignore

@@ -1,7 +1,6 @@
 vendor/pkg
 /runc
 /runc-*
-/contrib/cmd/memfd-bind/memfd-bind
 /tests/cmd/_bin
 man/man8
 release