Remove the invalid GID part from PartTable before starting PartTable migration

More people than we expected have invalid 'GID' part in their PartTable,
which breaks migration to schema 25, because it expects all part types
to have a valid name.

To work around this fact, we DELETE all parts with name 'GID' from PartTable
before starting the actual migration. This will not fix the migration for
people with other invalid parts, but I haven't heard of any such. To make
this completely bullet-proof, we would need to iterate through all entries,
which would be massively slower than current INSERT INTO ... SELECT FROM approach.

Distributions, this is a good choice for backporting into 1.12.1 ;-)

BUG: 331867
FIXED-IN: 1.12.2