Rust: Don't depend on unused 'target-libffi', 'target-libbacktrace'

For example:

    configure: error: "libffi has not been ported to nvptx-unknown-none."

Follow-up to commit a75f038c
"gccrs: Add config-lang.in", which said:

> This was a copy/paste from gccgo front-end. We do not use any of the
> target_libs yet, [...]

	gcc/rust/
	* config-lang.in (target_libs): Remove.

gcc/rust/config-lang.in

@@ -29,6 +29,4 @@ compilers="rust1\$(exeext)"
 
 build_by_default="no"
 
-target_libs="target-libffi target-libbacktrace"
-
 gtfiles="\$(srcdir)/rust/rust-lang.cc"