Remove unnecessary Integer.parse/1

Now that we have introduced Integer.parse/2 a few days ago, we can get rid of Integer.parse/1. Instead, we can specify a default base, just as we only have Integer.do_digits/2 (and Integer.do_undigits/2) with default base.

This PR may have a merge conflict with PR https://github.com/elixir-lang/elixir/pull/2971 if both look good.