Convert rust_libraries and rust_stdlinkage mutators to TransitionMutators

Replace rust.LibraryMutator and rust.LibstdMutator with
TransitionMutators.

Bug: 319288033
Flag: EXEMPT refactor
Test: all soong tests pass
Test: no change to build.ninja
Change-Id: Ia24a582119d39889279d7b93bac9259685153619
diff --git a/rust/coverage.go b/rust/coverage.go
index e0e919c..91a7806 100644
--- a/rust/coverage.go
+++ b/rust/coverage.go
@@ -47,7 +47,7 @@
 
 		// no_std modules are missing libprofiler_builtins which provides coverage, so we need to add it as a dependency.
 		if rustModule, ok := ctx.Module().(*Module); ok && rustModule.compiler.noStdlibs() {
-			ctx.AddVariationDependencies([]blueprint.Variation{{Mutator: "rust_libraries", Variation: "rlib"}, {Mutator: "link", Variation: ""}}, rlibDepTag, ProfilerBuiltins)
+			ctx.AddVariationDependencies([]blueprint.Variation{{Mutator: "rust_libraries", Variation: "rlib"}}, rlibDepTag, ProfilerBuiltins)
 		}
 	}