rust: Remove rust_ffi_rlibs, keep rust_ffi_static

rust_ffi_static and rust_ffi_rlibs both alias to rust_library_rlib now.
We need to think about whether we need these aliases at all.

There are no users of rust_ffi_rlibs at the moment as developers are
continuing to declare rust_ffi_static modules. Let's remove this alias
in the meantime at least to simplify things.

Bug: 350633786
Bug: 383552450
Test: m blueprint_tests && m rust
Change-Id: I6be895ddbffc5ac6303f2edc9ab20f7c20cd1591