Remove IsDependencyRoot from interface

This is equivalent to Binary() -- reduce the interface and improve
clarity.

Test: go test soong tests
Change-Id: I770f5ce79fd4d888586d31ec5e67be88153626b6

rust/binary.go

@@ -155,7 +155,3 @@ func (binary *binaryDecorator) stdLinkage(ctx *depsContext) RustLinkage {
 	}
 	return binary.baseCompiler.stdLinkage(ctx)
 }
-
-func (binary *binaryDecorator) isDependencyRoot() bool {
-	return true
-}