hostnet: drop vestigial dune-release watermarks from cohttp_mirage_io

The %%NAME%% %%VERSION%% watermarks (copied from cohttp) made `dune subst`
fail with "repository does not contain any version information" during the
Docker builds (bin-image and Dockerfile.test), which run the {dev} build with
no git version metadata. vpnkit doesn't use dune-release watermarking, so just
remove them. Pre-existing failure on master, not specific to this branch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: David Scott <dave@recoil.org>

Author: djs55

src/hostnet/cohttp_mirage_io.ml

@@ -14,7 +14,6 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *
- * %%NAME%% %%VERSION%%
  *)
 
 open Lwt.Infix

src/hostnet/cohttp_mirage_io.mli

@@ -14,7 +14,6 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *
- * %%NAME%% %%VERSION%%
  *)
 
 (** Cohttp IO implementation using Mirage channels. *)