Surround CheckRelation[Oid]LockedByMe() with USE_ASSERT_CHECKING

Hi hackers,

While working on a rebase for [1] due to 0cecc908e97, I noticed that
CheckRelationLockedByMe() and CheckRelationOidLockedByMe() are used only in
assertions.

I think it would make sense to declare / define those functions only for
assert enabled build: please find attached a tiny patch doing so.

Thoughts?

[1]: https://www.postgresql.org/message-id/flat/ZiYjn0eVc7pxVY45%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com