fixes broken discriminators of float types by disabling it

[ringabout]

type
  Case = object
    case x: float
    of 1.0:
      id: int
    else:
      ta: float

It segfaults with fatal error: invalid kind for firstOrd(tyFloat)

It was caused by #12591 and has affected discriminators of float types since 1.2.x

I think no one is using discriminators of float types anyway so I simply disable it like what was done to discriminators of string types (ref #15080)

ref nim-lang/nimony#1069