cargo: Remove *_stub from workspace member
cargo: Remove *_stub from workspace member

libcras_stub and system_api_stub are empty replacements for libcras and
system_api respectively. so cargo is happy even if they're missing

BUG=none
TEST=cargo build with CL:3267568

Change-Id: I20ba2ca77a241e354a45816605282d06d26f895f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3267907
Auto-Submit: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

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