make: Don't rustdoc libraries from Rust's stdlib

For reasons that aren't clear, as of Rust 1.82, running rustdoc on
these libraries cause presubmit failures when building Trusty with
Soong. We do not need to ship documentation for these libraries as
they are not particular to Trusty.

Bug: None
Test: Treehugger
Change-Id: Iaf8a7202dbe3ed9df129e16868a813d86fe9f2aa