Remove IsDependencyRoot from interface

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

Test: go test soong tests
Change-Id: I770f5ce79fd4d888586d31ec5e67be88153626b6

rust/compiler.go

@@ -289,10 +289,6 @@ func (compiler *baseCompiler) CargoOutDir() android.OptionalPath {
 	return android.OptionalPathForPath(compiler.cargoOutDir)
 }
 
-func (compiler *baseCompiler) isDependencyRoot() bool {
-	return false
-}
-
 func (compiler *baseCompiler) strippedOutputFilePath() android.OptionalPath {
 	return compiler.strippedOutputFile
 }