Revert "math.fish: support floating-point maths"

This reverts commit 7cad0069e8f4a652d0e22f1b0198f67399e80157.

https://github.com/fish-shell/fish-shell/issues/1723
http://superuser.com/questions/31445/gnu-bc-modulo-with-scale-other-than-0

This is why we can't have nice things.