security/sssd2: Use SAMBA_PORT and SAMBA_LDB_PORT

security/sssd2 can depend on any Samba version we have in the ports
tree. However, having the exact version of Samba hardcoded in *_DEPENDS
variables makes it hard to use a non-default Samba version.

Let the port pick up the default Samba version (and its ldb port) via
samba.mk. Since net/samba416 is still the default, nothing really
changes for sssd2. However, users can now build sssd2 against
net/samba419 if they set DEFAULT_VERSIONS=samba=4.19.

PR: 278839 280774
Approved by: portmgr (infrastructure)
Sponsored by: Klara, Inc.
Co-authored-by: Xavier Beaudouin <kiwi@oav.net>