Obsolete since 4bc2761a. This struct
existed so copying a passive grab could be simply done by
  activeGrab = *grab

and thus have a copy of the GrabPtr we'd get from various sources but still
be able to check device->grab for NULL.

Since 4bc2761a

 activeGrab is a pointer itself and points to the same memory
as grabinfo->grab, leaving us with the potential of dangling pointers if
either calls FreeGrab() and doesn't reset the other one.

There is no reader of activeGrab anyway, so simply removing it is
sufficient.

Note: field is merely renamed to keep the ABI. Should be removed in the
future.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 5363433a)