[clang][tests] remove darwin target from c tests

Summary:
This target does not work on linux unless there are cross-compilation headers
available for apple-darwin14.

infer/tests/utils/InferRunner.java

@@ -200,7 +200,6 @@ public class InferRunner {
     }
     ImmutableList<String> clangCmd = new ImmutableList.Builder<String>()
         .add("clang")
-        .add("--target=x86_64-apple-darwin14")
         .add("-x")
         .add(getClangLangOption(lang))
         .add(getStdParam(lang))