[rust] Clean up unused link variations.

rust_library modules no longer produce "link" variants alongside
"rust_libraries" variants as the former have been moved to rust_ffi
modules. rust_library modules however still create empty link variants
and code paths still assume that modules can support both rust linkage
and cc linkage. This adds unnecessary complexity that no longer serves a
purpose.

This cleans this up by removing the unused "link" variant for
rust_library modules and simplifies code paths that can now assume
modules are either a rust library or a cc library, but not both.

This also fixes a bug where Shared() was returning the wrong value.

Bug: 159064919
Test: cd external/rust/; mma
Test: cd external/crosvm/; mma

Change-Id: I3b5498c80b315c56b621dcb1388022fecc1bfc1b