kbuild: delete redundant LDSCRIPT definition

$(SRCTREE)/$(CPUDIR)/u-boot.lds is our default location
of arch-specific linker script.

Remove redundant definitions in
arch/{arc,microblaze,openrisc}/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Acked-by: Michal Simek <monstr@monstr.eu>