Remove ScalarType Xml

[mavilein]

We decided to remove the scalar type Xml because it does not add real value.
This means that the following schema is not possible anymore:

model Blog {
  id  Int @id
  xml Xml
}

Instead users will have to write it like this:

model Blog {
  id  Int @id
  xml String @mydb.Xml
}

[mavilein]

Tasks:

• adapt parser
• adapt introspection
• do not touch query engine with PrismaValue etc. We want to do this to save some effort.
• adapt ME (might be needed)