[fuchsia] Remove zx_socket_shutdown call

This system call is being transitioned to zx_socket_set_disposition.
However, in this particular usage the call is superfluous; closing the
socket has the same effect as shutting down writes on it since this
socket is never duplicated. Remove the comment which repeats the code
rather than update it.

See https://fxbug.dev/81452 for details on the system call migration.

Fixed: 1243099
Change-Id: I511c2fdb4bef5c7c5327871948e20d56336e5c73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3139656


Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Tamir Duberstein <tamird@google.com>
Cr-Commit-Position: refs/heads/main@{#918749}