Remove system-of-record APIs from migrate

Problem/Motivation

In the Drupal 7 contrib migrate module, there was a "system-of-record" concept. By default migrating into an existing content item completely replaces it with source data (system-of-record == SOURCE). In D7, setting the system-of-record to DESTINATION allowed you to selectively import only particular fields from the source, leaving other fields (which may have been manually modified on the destination side) unmolested. APIs for the system-of-record support (getSystemOfRecord(), setSystemOfRecord()) got carried forward into D8 but were never actually used - instead, we implemented an overwrite_properties property on destination plugins which achieves the same goal more cleanly. The vestigial system-of-record stuff should be removed.

Proposed resolution

1. Remove getSystemOfRecord() and setSystemOfRecord() from MigrationInterface and Migration.
2. Remove the SOURCE and DESTINATION constants from MigrationInterface.
3. Remove the protected $systemOfRecord property from Migration.

Remaining tasks

Do it.

User interface changes

N/A

API changes

getSystemOfRecord() and setSystemOfRecord() removed from the interface. Since these are never actually used anywhere, and it's highly likely anyone is implementing MigrationInterface other than via the existing Migration plugin class, the likely impact of removing them is zero.

Data model changes

None.