mfi-common: Drop freebsd builds on non-freebsd flights

mfi-common: Drop freebsd builds on non-freebsd flights

Something is wrong with the freebsd repositories.  This causes freebsd
host installation to fail.  This problem has persisted for a while now
and is blocking other work.

For now, drop these.  The jobs being dropped are
      build-amd64-xen-freebsd
      build-amd64-xen-xsm-freebsd
in the branches
      osstest
      xen-unstable
(verified with standalone-generate-dump-flight-runvars).

CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>