drm/amdkfd: Remove unused functions

kfd_device_by_pci_dev(), kfd_get_pasid_limit() and kfd_set_pasid_limit()
have been unused since 2023's
commit c99a2e7a ("drm/amdkfd: drop IOMMUv2 support")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>