Makefile: drop recursive libcacard clean

Commit eb8eb53e5846a957cf333f2e1ec8cb6e0c04 ("libcacard: rewrite
Makefile in non-recursive style") refactored libcacard/Makefile so it
can be included by the top-level Makefile.

The top-level clean target still loops over subdirectories, including
libcacard/, to invoke recursive clean.  Remove libcacard from the
recursive clean since its files are already included at the top level.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
