Floss: Do not use tokio feature memchr

Library tokio feature memchr is not used, and memchr is not supported on
latest tokio. Remove tokio in system/gd/rust/shim/Cargo.toml as it's not
used.

Bug: 262928525
Tag: #floss
Test: ./build.py --target utils
Merged-In: I4ced6887bf32f3a15d5554ae9a865f42fc0b30e1
Change-Id: I4ced6887bf32f3a15d5554ae9a865f42fc0b30e1