Reset information about modified or added items in subscription context

References

This PR resets information about modified or added items in subscription context. (see #380)

See #380 for more information about the bug, which is resolved here.

List of changes in this PR:

[x] My PR is small in size (e.g. less than 1,000 lines of code, not including comments & integration tests). Exceptions may be made if previously agreed upon.
[ ] My PR passes Checkstyle validation based on the Code Style Guide.
[ ] My PR includes Javadoc for all new (or modified) public methods and classes. It also includes Javadoc for large or complex private methods.
[ ] My PR passes all tests and includes new/updated Unit or Integration Tests based on the Code Testing Guide.
[ ] If my PR includes new libraries/dependencies (in any pom.xml), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
[ ] If my PR modifies REST API endpoints, I've opened a separate REST Contract PR related to this change.
[ ] If my PR includes new configurations, I've provided basic technical documentation in the PR itself.
[x] If my PR fixes an issue ticket, I've linked them together.