board/mpl: Remove mpl-specific memory test command

The mpl-specfic memory test is only documented for one board, doesn't
compile cleanly, uses improper coding style, and overlaps functionality
with U-Boot's common 'mtest' command, so lets get rid of it.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
CC: d.peter@mpl.ch
CC: d.mueller@elsoft.ch
CC: wd@denx.de