Remove checking detached buffer of a validated typed array in
%TypedArray%.prototype.slice. Now, JSTypedArray::Validate checks
if the new typed array's buffer is detached or not.

Bug: v8:5929
Change-Id: I381e33e3995ae10cc2907a7141a64ec9f97c742f
Reviewed-on: https://chromium-review.googlesource.com/721399Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48818}