[Hexagon] Remove ValidArch (NFC)

This function seems to be unused for at least one year.

llvm/lib/Target/Hexagon/HexagonArch.h

@@ -18,12 +18,6 @@
 namespace llvm {
 namespace Hexagon {
 
-template <class ArchCont, typename Val>
-bool ValidArch(ArchCont const &ArchList, Val HexArch) {
-  return std::any_of(std::begin(ArchList), std::end(ArchList),
-                     [HexArch](Val V) { return V == HexArch; });
-}
-
 template <class ArchCont, typename Val>
 llvm::Optional<ArchEnum> GetCpu(ArchCont const &ArchList, Val CPUString) {
   llvm::Optional<ArchEnum> Res;