MIPS: DEC: remove unbuildable promcon.c

promcon.o is built if CONFIG_PROM_CONSOLE is set. But there's no Kconfig
symbol PROM_CONSOLE, so promcon.c is unbuildable. Remove it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/5344/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>