Summary:
b263910ba90dd273d10a8ecffa90b8affcdea529 / D36731 moved ybctid out of ItemPointerData. While doing so, it added a YB_TODO in unique_key_recheck() related to the ybctid argument of index_insert() invocation.

unique_key_recheck() is only invoked for deferrable unique constraints, which are not yet supported in Yugabyte. So this function is not reachable as of today. Still, instead of passing 0 as ybctid, get the ybctid from the appropriate slot just like the itempointer. Naturally, this change cannot be tested now.
Jira: DB-16650

Test Plan:
Run pre-existing tests.

Close: #27163

Reviewers: jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D43924