drm/i915: Remove I915_READ64 and I915_READ64_32x2

Now that all their users are gone we can remove the macros and
accompanying duplicated comment.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610120608.15477-6-tvrtko.ursulin@linux.intel.com