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]

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>