Remove costly assert from RandIntGen

[simonster]

Restrict the type parameters instead.

Before:

julia> @time for i = 1:10000000; rand(1:4); end
elapsed time: 1.144407975 seconds (160000000 bytes allocated, 5.39% gc time)

After:

julia> @time for i = 1:10000000; rand(1:4); end
elapsed time: 0.259933851 seconds (0 bytes allocated)

cc @ivarne @rfourquet

[JeffBezanson]

How did that get in there?

[kmsquire]

@ivarne, this might have been the right thing to do, but I think @JeffBezanson was considering reverting #8309 instead. (He could have been clearer about that.)

[ivarne]

I read thing in the wrong order, so I didn't see it early enough. This is reverted together with #8309 in 6117328