crypto/internal/boring,crypto/sha1: remove cmd_go_bootstrap build tag

crypto/internal/boring,crypto/sha1: remove cmd_go_bootstrap build tag

Since CL 402595, the Go compiler no longer uses any package under
crypto, so there is no need to explicitly exclude boring from the
go bootstrap build.

Change-Id: Ib71349fffaab151c6e1fb42a9684151439b70cc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/508402
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>