[IA64] Removed "task_size" element from thread_struct - it is now constant

When the 32-bit compat code was deleted, we should also have removed
the task_size element from the thread structure - threads can only
be 64-bit now, so no need to keep track of how much virtual address
space each task can have ... everyone gets 0xa000000000000000.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tony Luck <tony.luck@intel.com>