sparc/console: kill off obsolete declarations

commit 09d3f3f0 ("sparc: Kill PROM console driver.") missed removing
the declarations of the deleted prom_con structure and prom_con_init
function from console.h. Kill them off now.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>