Revert "make perl core quiet under -Wfloat-equal"

A suggested way of avoiding the the warning on nv1 != nv2
by replacing it with (nv1 < nv2 || nv1 > nv2), has too many issues
with NaN.  [perl #120538].

I haven't found any other way of selectively disabling the warning,
so for now I'm just reverting the whole commit.

This reverts commit c279c4550ce59702722d0921739b1a1b92701b0d.