Remove unused BumpBlockIsValid macro

Remove unused BumpBlockIsValid macro

The bump allocator was recently added in 29f6a959c.  Our other
allocators have a similar macro to this, but seemingly the version of
the macro for those allocators is only used in places where the chunk
header is decoded.  Since the bump allocator has no chunk header, none
of those functions exist for bump therefore macro is unused.  Remove it.

Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/5f724fb2-96e1-4f36-b65b-47b337ad432e@eisentraut.org