[Fuchsia] Remove unused header from SDK GN rules.

The fdio/socket.h header is unused, and an upcoming SDK roll will
remove it, so remove it from our hand-rolled build-rules.

Change-Id: I84260f09778daac364fd217198292b3272cf38a4
Reviewed-on: https://chromium-review.googlesource.com/c/1287294
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600806}

third_party/fuchsia-sdk/BUILD.gn

@@ -41,7 +41,6 @@ fuchsia_sdk_pkg("fdio") {
     "include/lib/fdio/module.modulemap",
     "include/lib/fdio/namespace.h",
     "include/lib/fdio/remoteio.h",
-    "include/lib/fdio/socket.h",
     "include/lib/fdio/spawn.h",
     "include/lib/fdio/unsafe.h",
     "include/lib/fdio/util.h",