[SYCL][NFC] Code cleanup revealed by self-build. (#3153)

Remove redundant false case in switch statement following:
https://llvm.org/docs/CodingStandards.html#don-t-use-default-labels-in-fully-covered-switches-over-enumerations

Author: zahiraam

llvm/tools/sycl-post-link/sycl-post-link.cpp

@@ -364,8 +364,6 @@ static void collectKernelModuleMap(
         // the map key is not significant here
         ResKernelModuleMap["<GLOBAL>"].push_back(&F);
         break;
-      default:
-        llvm_unreachable("unknown scope");
       }
     }
   }