[BE] Remove unused `export.h` include (#84305)
As flatbuffer_serializer can be compiled without it
Found while debugging cause of https://github.com/pytorch/pytorch/pull/82040#issuecomment-1229503604
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84305
Approved by: https://github.com/kit1980, https://github.com/qihqi
