sparc_brk() is not needed anymore

the checks it's doing are duplicated in sys_brk() and failing
them early makes no sense, AFAICT.

Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>