[VFABI] Remove unused variables in testcase, fix buildbot

E.g. the buildbot at

 http://lab.llvm.org:8011/builders/ppc64le-lld-multistage-test/builds/7259/steps/build-stage2-unified-tree/logs/stdio

failed with

/home/buildbots/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/unittests/Transforms/Utils/VFABIUtils.cpp:50:22: error: unused variable 'FnAttrs' [-Werror,-Wunused-variable]
  const AttributeSet FnAttrs = Attrs.getFnAttributes();
                     ^
1 error generated.

Author: mikaelholmen

llvm/unittests/Transforms/Utils/VFABIUtils.cpp

@@ -46,8 +46,6 @@ TEST_F(VFABIAttrTest, Write) {
   Mappings.push_back("_ZGVnN8v_g");
   Mappings.push_back("_ZGVnN2v_g(custom_vg)");
   VFABI::setVectorVariantNames(CI, Mappings);
-  const AttributeList Attrs = CI->getAttributes();
-  const AttributeSet FnAttrs = Attrs.getFnAttributes();
   const StringRef S = CI->getAttribute(AttributeList::FunctionIndex,
                                        "vector-function-abi-variant")
                           .getValueAsString();