Enum.sample/1, 2 should raise if the collection size is smaller than the sample size

This fixes the Enum.sample/1 issue discussed in the issue #2916.

And addresses @lexmag's comment in the closed PR #2917.
Actually, someone pointed out it is best to not raise if the sample is not large enough. That's because it is easier for the user to check if the sample is large enough *afterwards* since checking before hand can be hard if you have lazy collections.

So you can you please remove this particular part? Everything else in the PR looks great. :) Thank you!