Remove data about model entities/fields removed in Che6 from Che DB

[garagatyi]

Description

We have plenty of data from Che5 which are not needed in Che6. We should remove them if we don't want to wipe the old DB.
Some of the entities to remove:

• MachineSourceImpl
• ServerConfImpl (or ServerConf2Impl)

Reproduction Steps

OS and version:

Diagnostics:

[mkuznyetsov]

while using Old Che 5.22.1 and the upgrading to Che 6, I coudn't find neither MachineSourceImpl nor old ServerConfImpl objects in database. I would close this one, if there are no clear reproduction steps for it