devices: vvu-proxy: Remove Error::RxDescriptorsExhausted
devices: vvu-proxy: Remove Error::RxDescriptorsExhausted

Remove `Error::RxDescriptorsExhausted`, which was matched against.
As a result, no variants in `Error` enum appears in pattern matching and
we can switching to `anyhow::Error` easily in a following CL.

BUG=b:232273004
TEST=cargo build

Change-Id: Ie0fb0897b7ec09d901011d7d9e62af3a00b08bc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3640876
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Auto-Submit: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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