db-switchover: Only configure semisync once

Currently if you run db-switchover --only-slave-move ... and then
db-switchover --skip-slave-move ..., semisync will be configured twice
on the new primary. This seems to be triggering some bug in mariadb, and
is anyway unnecessary. So, instead only do it once.

Bug: T288500
Change-Id: I2ae86636c864aec2cf59e9fd8845221902860181