Remove newly added asserts from pg_bitutils.h

Remove newly added asserts from pg_bitutils.h

These were valuable during development, but are unlikely to tell us
anything going forward. This reverts 204b0cbec and adjusts the content
of 677319746 to more closely match the more-readable original style.

Per review from Tom Lane

Discussion: https://www.postgresql.org/message-id/3567481.1676906261%40sss.pgh.pa.us