remove invalid workaround for expf (glibc), 2X speedup

there was a workaround for x86_64 for glibc bug (4years ago),
where expf was slower than exp, so exp was used as a workaround.

that is not true anymore, use expf()  and code is 2X faster.

See and test: https://bugzilla.redhat.com/show_bug.cgi?id=521190