aconfig: remove reference to non-existent workspace member "aconfig_storage_write_api"

Commit bca2f3c2 incorrectly added "aconfig_storage_write_api" as a
workspace member even though that project has not yet been created. This
broke cargo.

Unbreak cargo by removing "aconfig_storage_write_api".

Bug: N/A
Test: croot build/make/tools/aconfig && cargo c
Change-Id: Ic0f71a53d49736228a376f64808aea24c2ec8ce5