chore: remove replaces sections from squashing migrations.

[pwnage101]

This enables --prune to function and actually delete old records in the migrations table.

(edxapp) www-data@ip-10-3-10-206:/edx/app/edxapp/edx-platform$ ./manage.py lms migrate --prune
...
CommandError: Migrations can be pruned only when an app is specified.
...
(edxapp) www-data@ip-10-3-10-206:/edx/app/edxapp/edx-platform$ ./manage.py lms migrate enterprise --prune
...
Pruning migrations:
  Cannot use --prune because the following squashed migrations have their 'replaces' attributes and may not be recorded as applied:
    social_django.0001_initial
    social_django.0004_auto_20160423_0400
    social_django.0005_auto_20160727_2333
    social_django.0002_add_related_name
    social_django.0003_alter_email_max_length
    sap_success_factors.0004_auto_20220324_1550_squashed_0006_auto_20220330_1157
    sap_success_factors.0001_squashed_0022_auto_20200206_1046_squashed_0011_alter_sapsuccessfactorslearnerdatatransmissionaudit_enterprise_course_enrollment_id
    blackboard.0001_initial_squashed_0014_alter_blackboardlearnerassessmentdatatransmissionaudit_enterprise_course_enrollment_id
  Re-run 'manage.py migrate' if they are not marked as applied, and remove 'replaces' attributes in their Migration classes.