drm/amdkfd: fix svm_migrate_fini warning

In the Linux kernel, the following vulnerability has been resolved:

drm/amdkfd: fix svm_migrate_fini warning

Device manager releases device-specific resources when a driver
disconnects from a device, devm_memunmap_pages and
devm_release_mem_region calls in svm_migrate_fini are redundant.

It causes below warning trace after patch "drm/amdgpu: Split
amdgpu_device_fini into early and late", so remove function
svm_migrate_fini.

BUG: https://gitlab.freedesktop.org/drm/amd/-/issues/1718

WARNING: CPU: 1 PID: 3646 at drivers/base/devres.c:795
devm_release_action+0x51/0x60
Call Trace:
? memunmap_pages+0x360/0x360
svm_migrate_fini+0x2d/0x60 [amdgpu]
kgd2kfd_device_exit+0x23/0xa0 [amdgpu]
amdgpu_amdkfd_device_fini_sw+0x1d/0x30 [amdgpu]
amdgpu_device_fini_sw+0x45/0x290 [amdgpu]
amdgpu_driver_release_kms+0x12/0x30 [amdgpu]
drm_dev_release+0x20/0x40 [drm]
release_nodes+0x196/0x1e0
device_release_driver_internal+0x104/0x1d0
driver_detach+0x47/0x90
bus_remove_driver+0x7a/0xd0
pci_unregister_driver+0x3d/0x90
amdgpu_exit+0x11/0x20 [amdgpu]