Remove unused xid parameter.
Commit
6c2003f8a1 removes the use of transaction id's for exporting
snapshots. This commit removes one unused xid parameter left behind in
SnapBuildGetOrBuildSnapshot.
Author: Melih Mutlu
Reviewed-By: Zhang Mingli
Discussion: https://postgr.es/m/CAGPVpCTqZRoDKgCycw+eYi+Gq41rN9pU-gntgTd7wfsNDpPL3Q@mail.gmail.com