cmd: mem: Remove already removed CONFIG_SYS_MEMTEST_SCRATCH

The commit e519f03a ("cmd: mem: Remove CONFIG_SYS_MEMTEST_SCRATCH
mapping") removed CONFIG_SYS_MEMTEST_SCRATCH but commit 09140113
("command: Remove the cmd_tbl_t typedef") has added it back. That's why
symbol is still in the tree that's why remove it again.

Fixes: 09140113 ("command: Remove the cmd_tbl_t typedef
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>