mpc8xx: remove unused linker script

Now TQM8xx is the only remaining board family of mpc8xx.
It uses its own linker script, board/tqc/tqm8xx/u-boot.lds.

arch/powerpc/cpu/mpc8xx/u-boot.lds is not used by any boards.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>