remove fallback rand method which triggers ambiguity warning
The point of the method rand(::AbstractRNG, ::Type{Close1Open2})
(from 76f0a3251) was to allow to plugin an arbitrary RNG into randn,
but this is ambiguous with
rand{I<:FloatInterval}(::MersenneTwister, ::Type{I}).
So rather let's other RNGs explicitly define their own such method.
