cmd: remove unused `display` command

Compiling the display command leads to an error

    undefined reference to `display_set'

No implementation of display_set() exists in U-Boot.

Eliminate the `display` command as well as the accompanying files.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>