[SYCL][NFC] Drop unnecessary nullptr check (#18917)

This resolves Coverity issue `440250`
(https://scan.coverity.com/projects/intel-llvm?tab=overview).

We know that `ND` cannot be `nullptr`, but presence of the check makes
Coverity think that earlier accesses (within the `while` loop condition)
are potentially unsafe.

Author: AlexeySachkov

clang/lib/Sema/SemaSYCL.cpp

@@ -846,7 +846,7 @@ static bool isDeclaredInSYCLNamespace(const Decl *D) {
     ND = cast<NamespaceDecl>(Parent);
   }
 
-  return ND && ND->getName() == "sycl";
+  return ND->getName() == "sycl";
 }
 
 static bool isSYCLPrivateMemoryVar(VarDecl *VD) {