sparc32: Kill totally unused memory information tables.

The code in arch/sparc/prom/memory.c computes three tables, the list
of total memory, the list of available memory (total minus what
firmware is using), and the list of firmware taken memory.

Only the available memory list is even used.

Therefore, kill those unused tables and make prom_meminfo() return
just the available memory list.

Signed-off-by: David S. Miller <davem@davemloft.net>