drop SASL support allowing using Debians libspice-server

drop SASL support allowing using Debians libspice-server

The disadvantage over keeping this and thus having to maintain
libspice-server ourself, a package with frequent security issues
reported, overweight the advantages. Spiceterm itself is probably not
used that much, and of that relatively small user base SASL is
probably almost non-existent.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>