Revert "Fix invalid link in libnftables-dev"

This reverts commit 9db9435a0a5f4362212b999d6afab4097c9c96e6.

If the `libnftables.so' file is missings, no other package that requires
libnftables-dev can be build.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>