Rolling back adding retention instructions to sample table

[dasunpubudumal]

Closes #633

Changes proposed in this pull request

• Removing adding the retention_instruction column in sample table.

This migration has already been applied in UAT warehouse. We can introduce a remove_column migration but that will cause problems when the two migrations add_column and remove_column goes into production as add_column in sample table is a huge task (as it is ~4Gi in size).

So the suggested change rollbacks the changes done for adding retention instructions to sample table, so that it will have no effect on the sample table in production. The migration added to UAT database is to be manually removed.

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