remove unused mkldnn include (#22217)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22217
Seems introduced in https://github.com/pytorch/pytorch/pull/19209/files which is
no longer used. Remove it to simplify mobile build.
Reviewed By: bddppq
Differential Revision: D15983344
fbshipit-source-id: 37ee0bfbd022da09af6bc44c6e3fec1c99a8e732
