dm: drop CONFIG_DM_DEVICE_REMOVE from uncmd list

We do not want to compile the DM remove code for SPL.  Currently,
we undef it in include/config_uncmd_spl.h (for C files) and in
scripts/Makefile.uncmd_spl (for Makefiles).  This is really ugly.

This commit demonstrates how we can deprecate those two files.

Use $(SPL_) for the entry in the Makfile and CONFIG_IS_ENABLED()
in C files.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>