Grab the low-hanging fruit from forcing USE_FLOAT8_BYVAL to true.

Grab the low-hanging fruit from forcing USE_FLOAT8_BYVAL to true.

Remove conditionally-compiled code for the other case.

Replace uses of FLOAT8PASSBYVAL with constant "true", mainly because
it was quite confusing in cases where the type we were dealing with
wasn't float8.

I left the associated pg_control and Pg_magic_struct fields in place.
Perhaps we should get rid of them, but it would save little, so it
doesn't seem worth thinking hard about the compatibility implications.
I just labeled them "vestigial" in places where that seemed helpful.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1749799.1752797397@sss.pgh.pa.us