MIPS: Remove dead CONFIG_MTD_PB1550_BOOT, CONFIG_MTD_PB1550_USER

CONFIG_MTD_PB1550_BOOT, CONFIG_MTD_PB1550_USER doesn't exist in
Kconfig, therefore removing all references for it from the source
code.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
To: Manuel Lauss <manuel.lauss@gmail.com>,
To: linux-mips@linux-mips.org
To: linux-kernel@vger.kernel.org
Cc: vamos@i4.informatik.uni-erlangen.de
Patchwork: https://patchwork.linux-mips.org/patch/1370/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>