MIPS: pb1x00: remove custom u-boot.lds script

Remove the board specific linker script. It is not
needed anymore, the unified MIPS linker script can
be used instead.

All pb1x00 targets are producing identical binary
images after the change than before.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>