base: unix: remove unused IntoRawDescriptor for PollContext
base: unix: remove unused IntoRawDescriptor for PollContext

PollContext is wrapped by the platform-agnostic WaitContext, which does
not support IntoRawDescriptor, so nobody should be using this.

BUG=None
TEST=cargo build

Change-Id: I5d931d20cb3d0a8f42cc1e625d35df4047e45c7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3687058
Reviewed-by: Anton Romanov <7c8c28e1283c1d87f2c3df4d6b5c74b6ae02bcd9@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm