The FakeServerMigration.get_server_migrations method was added in
change I15b4a5aca8d0dee59dd293e7b1c7272cdfbeea20 but has never been
used. Remove it.
Change-Id: I6089c5200737b9319a8e96f2a2fc18b7cdd6b2c6
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>