[mlir][spirv] Drop outdated TODO comment

[kuhar]

Summary

We do not plan to handle wide vector types with SPIR-V arrays anymore.

mlir/lib/Dialect/SPIRV/Transforms/SPIRVConversion.cpp

 convertVectorType(const spirv::TargetEnv &targetEnv,
                   const SPIRVConversionOptions &options, VectorType type,
                   std::optional<spirv::StorageClass> storageClass = {}) {
   auto scalarType = type.getElementType().cast<spirv::ScalarType>();
   if (type.getRank() <= 1 && type.getNumElements() == 1)
     return convertScalarType(targetEnv, options, scalarType, storageClass);
 
   if (!spirv::CompositeType::isValid(type)) {
-    // TODO: Vector types with more than four elements can be translated into
-    // array types.
     LLVM_DEBUG(llvm::dbgs() << type << " illegal: > 4-element unimplemented\n");
     return nullptr;
   }
 
   // Get extension and capability requirements for the given type.
   SmallVector<ArrayRef<spirv::Extension>, 1> extensions;
   SmallVector<ArrayRef<spirv::Capability>, 2> capabilities;
   type.cast<spirv::CompositeType>().getExtensions(extensions, storageClass);