Drop the default stanza from mac_permissions.xml

All non matching apps will simply receive the seinfo
label of "default" implicitly. No need to further
clarify things anymore with an explicit default stanza.

Change-Id: Ib7b01ee004775f24db9a69340a31784b967ce030
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>