kbuild: deb-pkg: remove the CONFIG_MODULES check in buildeb

When CONFIG_MODULES is disabled for ARCH=um, 'make (bin)deb-pkg' fails
with an error like follows:

  cp: cannot create regular file 'debian/linux-image/usr/lib/uml/modules/6.4.0-rc2+/System.map': No such file or directory

Remove the CONFIG_MODULES check completely so ${pdir}/usr/lib/uml/modules
will always be created and modules.builtin.(modinfo) will be installed
under it for ARCH=um.

Fixes: b611daae ("kbuild: deb-pkg: split image and debug objects staging out into functions")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>