Remove MMDEF Dependencies

[ikiril01]

We should remove any MMDEF dependencies in the schemas. Specifically, this is applies to the Field_Data field in the MalwareSubjectType, which makes use of the MMDEF v1.2 fieldDataEntry type.

[ikiril01]

This is due to the fact that the fieldDataEntry type has a mandatory references field which is not needed in MAEC. However, we can likely derive our own FieldDataType that will be largely compatible, and cover the following:

• Time window for which it is valid
• First observed date for the malware instance
• Relative prevalence in the wild
• Source(s) of the malware instance (e.g., spam email, drive by download URL, etc.)
• Geolocation information - where the malware instance has been seen