Revert "Test conversion of NaN between float4 and float8."

Revert "Test conversion of NaN between float4 and float8."

This reverts commit 55e0e458170c76c1a0074cd550a13ec47e38a3fa.
It's served its purpose of demonstrating what was wrong on
buildfarm member opossum.  We could consider putting some kind
of single-purpose hack into ftod() to make the test pass there;
but I don't think it's worth the trouble, since there are surely
many other places whether this platform bug could manifest.