mips: xburst: remove remainders of dead board

Commit 54e458de deleted qi_lb60 board support
because of the incompatible license issue.

There is no board with XBurst CPU.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>