Do not put sysroot into unfiltered_compile_flags

Now that we have dedicated sysroot variable, it is not necessary to have it in
unfiltered_compile_flags (actually, it's bad to have it here because it assumes
--sysroot= flag, and that doesn't work cross-platform).

RELNOTES: None.
PiperOrigin-RevId: 169384693
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppModel.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppModel.java
index 42a596c..b7a8e60 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppModel.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppModel.java
@@ -500,7 +500,7 @@
    */
   public static Supplier<ImmutableList<String>> getUnfilteredCompileFlagsSupplier(
       CcToolchainProvider ccToolchain, ImmutableSet<String> features) {
-    return () -> ccToolchain.getUnfilteredCompilerOptionsWithSysroot(features);
+    return () -> ccToolchain.getUnfilteredCompilerOptions(features);
   }
 
   /**