iommu: Remove functions that support private domain

After moving iommu_group setup to iommu core code [1][2] and removing
private domain support in vt-d [3], there are no users for functions such
as iommu_request_dm_for_dev(), iommu_request_dma_domain_for_dev() and
request_default_domain_for_dev(). So, remove these functions.

[1] commit dce8d696 ("iommu/amd: Convert to probe/release_device()
    call-backs")
[2] commit e5d1841f ("iommu/vt-d: Convert to probe/release_device()
    call-backs")
[3] commit 327d5b2f

 ("iommu/vt-d: Allow 32bit devices to uses DMA
    domain")

Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20200513224721.20504-1-sai.praneeth.prakhya@intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>