Kernel: Remove unused 'ImmutableBufferArgument'

Author: https://github.com/BenWiederhake
Commit: https://github.com/SerenityOS/serenity/commit/2a8baf95826
Pull-request: https://github.com/SerenityOS/serenity/pull/4965
Reviewed-by: https://github.com/bugaevc
Reviewed-by: https://github.com/emanuele6

Kernel/API/Syscall.h

@@ -234,12 +234,6 @@ struct MutableBufferArgument {
     SizeType size { 0 };
 };
 
-template<typename DataType, typename SizeType>
-struct ImmutableBufferArgument {
-    const DataType* data { nullptr };
-    SizeType size { 0 };
-};
-
 struct StringListArgument {
     StringArgument* strings {};
     size_t length { 0 };