Revert "[MPS] Add support for Custom Kernels (#100661)"
This reverts commit f39cda83d133cdd92e06e3d8cdd91340b43eb2c2.
Reverted https://github.com/pytorch/pytorch/pull/100661 on behalf of https://github.com/malfet due to Break internal builds, but also guarding dispatch_t define behind __OBJC__ guard is not a good practices ([comment](https://github.com/pytorch/pytorch/pull/100661#issuecomment-1540540002))
