[SYCL] Silence a "local variable is initialized but not referenced" warning; NFC (#3870)

Author: AaronBallman

clang/lib/Driver/Driver.cpp

@@ -4976,7 +4976,6 @@ class OffloadingActionBuilder final {
       return;
 
     // Let builders add host linking actions.
-    Action* HA = nullptr;
     for (DeviceActionBuilder *SB : SpecializedBuilders) {
       if (!SB->isValid())
         continue;