TRUNK-6454: Remove the discriminator from OrderSetMember Hibernate Mapping

[wikumChamith]

Description of what I changed

This resolves the following error.

Caused by: org.hibernate.MappingException: Column 'order_set_member_id' is duplicated in mapping for entity 'org.openmrs.OrderSetMember_AUD' (use '@Column(insertable=false, updatable=false)' when mapping multiple properties to the same column)

Issue I worked on

see https://openmrs.atlassian.net/browse/TRUNK-6454

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.

  No? Unsure? -> configure your IDE, format the code and add the changes with git add . && git commit --amend

• I have added tests to cover my changes. (If you refactored
  existing code that was well tested you do not have to add tests)

  No? -> write tests and add them to this commit git add . && git commit --amend

• I ran mvn clean package right before creating this pull request and
  added all formatting changes to my commit.

  No? -> execute above command

• All new and existing tests passed.

  No? -> figure out why and add the fix to your commit. It is your responsibility to make sure your code works.

• My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master