drm/i915/gt: Drop i915_address_space::file (v2)

There's a big comment saying how useful it is but no one is using this
for anything anymore.

It was added in 2bfa996e ("drm/i915: Store owning file on the
i915_address_space") and used for debugfs at the time as well as telling
the difference between the global GTT and a PPGTT.  In f6e8aa38
("drm/i915: Report the number of closed vma held by each context in
debugfs") we removed one use of it by switching to a context walk and
comparing with the VM in the context.  Finally, VM stats for debugfs
were entirely nuked in db80a129 ("drm/i915/gem: Remove per-client
stats from debugfs/i915_gem_objects")

v2 (Daniel Vetter):
 - Delete a struct drm_i915_file_private pre-declaration
 - Add a comment to the commit message about history

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-24-jason@jlekstrand.net