microblaze: remove the explicit removal of system.dtb

I guess

    || (rm -f $@ && echo false)

... should be

    || (rm -f $@ && false)

In fact, no Makefile needs to delete a target explicitly on error.

It is automatically done since commit 9c2af1c7 ("kbuild: add
.DELETE_ON_ERROR special target").

I also reused equivalent cmd_shipped from scripts/Makefile.lib.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>