Remove deprecated methods after OSCALModel implementation

[meganwolf0]

Describe what should be investigated or refactored

Remove old methods that are no longer valid with updated OSCALModel interface implementation, for instance:

• WriteOscalModel
• MergeOscalModels
• (others?)

This issue can also contain the swap of the GetType method in the OSCALModel interface to instead of returning the string value for an oscal model, to assign const iota and return, as this is believed to be a cleaner, more idiomatic approach

Additional context

Blocked by #747, #748, #873

Expected Deliverable

• Old code and tests removed
• Updated OSCALModel interface definition