CA-109013: Revert "Making the check for both types of migrations"

This check doesn't work as is in the cross-pool case, since
the destination host won't be in the local database.

This reverts commit b79b9d0779a903911aa9e33796d3a127102848f4.
Support remote hosts in Helpers.version_string_of

Signed-off-by: John Else <john.else@citrix.com>
Add platform version check to cross-pool migration

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1311 from johnelse/ca-109013

CA-109013: Fix cross-pool migration and reimplement platform_version check