Cleanup INFINITY code in float.h

Cleanup INFINITY code in float.h

The INFINITY macro is always defined per C99 standard, so this should
mean we can now get rid of the workaround code for when that macro isn't
defined.

Also, delete the (now unneeded) #pragma code which was disabling a
compiler warning in MSVC.  There was a comment explaining why the #pragma
was placed outside the function body to work around a MSVC compiler bug,
but the link explaining that was dead, as reported by jian he.

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: jian he <jian.universality@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CACJufxGARYETnNwtCK7QC0zE_7gq-tfN0mME=gT5rTNtC=VSHQ@mail.gmail.com