Remove FLOATING_POINT from config.h.

This isn't used anywhere in the code, floating point is just the
default unless FIXED_POINT is defined.

In the speex codebase, arch.h has a check that both FIXED_POINT
and FLOATING_POINT aren't defined simultaneously, in part as a
check that the build system was constructed with thought about
these and other defines. However, we don't have such a check
and to me it seems unnecessary code.